Microbiology Final Exam
Covers Chapters 13, 20, 21, 24 and 25
Terms in this set (156)
Shift vs. Drift
Shift is caused by large changes of the genome (specifically the genes that code for antigens)--this generally leads to larger than normal outbreaks. drift is caused by smaller point mutations in the same genes. this causes the normally seasonal flu outbreaks.
How are lysogenic prophages reactivated?
Phages are activated most commonly by damage to the host bacterium. The stress on the organism causes the reactivation. UV light can damage DNA, thus activating the phage.
How does chlamydia cause symptoms?
Chlamydial infection results in the destruction of host cells by the immune system. This destruction is what causes inflammation and damage at the site of the infection.
A 20-year-old male reports to his physician that he has experienced painful urination, as if he were urinating molten solder. He has also noticed a pus-like discharge from his penis. The patient reports having been sexually active with two or three women in the previous six months. Because his partners reported being "absolutely sure" that they wouldn't get pregnant and carried no sexually transmitted diseases, the patient had not used a condom.
Questions as followed:
What disease does this patient have? What is the common name for this disease?
gonorrhea; " the clap"
How was this disease transmitted?
This is most commonly transmitted by unprotected sex.
Based on the recommendations in this textbook, which antibiotic would be the treatment of choice for this disease?
Virulence factors associated with this organism include all of the following, EXCEPT __________.
Neisseria gonorrheae does not have flagella.
Why did his sexual partners believe they were not infected with a STD?
Clinical signs/symptoms in females are often not detectable, especially early in the infection.
-PID, ectopic pregnancy, and infertility occur later in the course of the disease.
__________ are the primary means by which Neisseria gonorrhoeae attaches to the mucous membranes of the reproductive tract.
Who do you think would be more likely NOT to recognize the symptoms of gonorrhea: a man or a woman? Why?
Because of her particular anatomy, the normal discharge associated with the female reproductive tract, plus the fact that more women are asymptomatic for gonorrhea, a woman would be less likely to recognize the symptoms as being gonorrhea and seek treatment.
Even though Escherichia coli can cause, on occasion, severe human disease, there is no vaccine developed for it. Why do you think this is?
There is no vaccine developed for Escherichia coli because it is an advantageous part of our normal intestinal flora.
Of the pathogenic forms of Escherichia coli, the strain O157:H7 is the primary cause of hemorrhagic colitis, or bloody diarrhea, and can progress to the often fatal hemolytic uremic syndrome (HUS). What is the main reason for the severity of illness from this strain of E. coli?
The main reason for the severity of illness comes from the ability of this particular strain of E. coli to produce Shiga-like toxin, which causes an intense inflammatory response in the gut.
T or F: Humans are the sole hosts of Salmonella typhi.
How is pneumonic plague transmitted?
Inhalation of respiratory aerosols from a person infected with Yersinia pestis.
Bordetella pertussis causes disease by
interfering with the action of the ciliated epithelial cells of the trachea.
Two patients - a woman and her husband, ages 23 and 22, respectively - arrive at the health clinic one morning. They report having had severe abdominal cramps, grossly bloody diarrhea, nausea, and fever for 48 hours. Cultures of stool samples grown under microaerophilic, capneic conditions contain comma-shaped, Gram-negative bacilli. Both the patients are lactovegetarians and report being part of a "cow leasing" program at a local dairy in which patrons lease part of a cow's milk production so that they can drink natural, whole, raw milk. The couple devised the program so that they and several neighbors could circumvent state regulations prohibiting the sale of unpasteurized milk. Investigators obtained and cultured a milk sample from the dairy's bulk milk tank:
Questions as Followed:
What is the most likely cause of this couple's disease?
Campylobacter jejuni is the most common cause of bacterial gastroenteritis in the United States.
How did this couple become infected with C. jejuni?
ingestion of contaminated, raw milk
What is the word used to refer to diseases transmissible from animals to humans?
A zoonosis is a disease transmitted from animals to humans.
The signs and symptoms of infection with Helicobacter pylori are the result of
invasion by H. pylori into the mucosa of the stomach.
How are viruses different from cells?
They require a host in order to reproduce.
What is the function of the structural elements of a virus?
To package and protect the viral genome
Lysogenic viral DNA integrating into the host genome is referred to as
Which of the following events might trigger induction of a temperate bacteriophage?
Exposure to UV light
How is the lytic cycle different from the lysogenic cycle with respect to the infected host cell?
The host cell dies during the lytic stage.
What is the fate of the prophage during the lysogenic stage?
It is copied every time the host DNA replicate
In which stage is the viral DNA introduced into the cell?
In which stage does formation of mature viruses occur?
The host DNA is usually degraded during which stage?
What would be the fate of a lytic bacteriophage if the host cell died prior to the assembly stage?
The virus would not be able to infect new hosts.
The Davises were excited about their newborn twin boys and couldn't wait to take them to see Mr. Davis's father. Grandfather Davis was excited to see his first grandsons as well and thought their visit might help take his mind off the pain of his shingles, which had suddenly appeared only days before.
Which virus is responsible for Grandfather Davis's shingles?
The varicella-zoster virus is the cause of both chicken pox in the young and shingles in the elderly (people who recovered from chicken pox decades earlier).
Which nucleic acid is part of the varicella-zoster virion?
Is Grandfather Davis contagious?
Yes. The twins would probably develop chicken pox 2 weeks after their visit with Grandfather Davis
What is the recommended schedule for the first chicken pox vaccine in infants?
12 months of age
What type of vaccine would be used to vaccinate the twins?
attenuated virus vaccine
Which of the following families of DNA viruses is the most prevalent in humans?
What conditions may trigger reactivation of latent herpesvirus infections?
Fever, stress, immune suppression, or hormonal changes can all trigger reactivation.
A 25-year-old male was admitted to the hospital with thrush, diarrhea, unexplained weight loss, and difficulty in breathing. Cultures of pulmonary fluid revealed the presence of Pneumocystis jiroveci. The man admitted to being a heroin addict and to sharing needles in a "shooting gallery."
What additional virus should this man be tested for?
Based on the clinical signs and symptoms of this patient, what would you predict his CD4 helper T cell count to be?
A CD4 helper T cell count of <200/μl of blood is associated with AIDS.
How did this patient most likely acquire the HIV infection?
HIV was transmitted to this patient through IV drug use in the "shooting gallery."
What organism causes thrush?
An opportunistic infection caused by Candida albicans.
Pneumocystis pneumonia is caused by an opportunistic __________.
A patient comes to his primary care provider very ill with an infection. The primary care provider would be most likely to suspect HIV/AIDS if the patient was diagnosed with which of the following infections?
Which of the following statements is true?
-It is possible to have AIDS without being infected with HIV.
-It is impossible to have HIV and AIDS at the same time.
-It is possible to be infected with HIV without having AIDS.
-Patients who are HIV positive but are not actively ill are not able to infect others.
It is possible to be infected with HIV without having AIDS
Why is it important to get a new flu vaccine every year?
Every season, the hemagglutinin and neuraminidase proteins on the viral envelope of influenza are subject to slight changes by mutation, allowing evasion of the immune system.
Many of the most severe strains of influenza A have originated in Asia. Asia is a major originator of pandemic strains and a major site of antigenic shift. Which of the following reasons could be a cause of this?
In Asia, the population density of humans, domesticated birds, and pigs is very high.
Neisseria gonorrhoeae (Gram Negative Cocci)
Symptoms: In men, insufferably symptomatic. Acute inflammation in 2-5 days, causing extremely painful urination & purulent discharge. In women, typically asymptomatic. Usually think its bladder infection or yeast infection.
Pelvic Inflammatory Disease: In the uterine tubes they can trigger inflammation, fever and abdominal pain. Can lead to scarring of the tubes and sterility or ectopic pregnancies.
Pathogenicity of Neisseria gonorrhooeae:
Virulence factors:Capsule, fimbraie and lipooligosaccharide adhere to human cells; IgA protease; can survive inside phagocytic cells; variable surface antigens.
Common member of the normal microbiota in upper respiratory tracts of up to 40% healthy people; common in children and YA in lower economic groups.
Symptoms: Abrupt sore throat, fever, headache, stiff neck, vomitting, convulsions. Occasionaly arthritis and partial loss of hearing occur. Can progress so rabidly that death can occur 6 hours after initial symptoms.
Neisseria meningitidis vaccination & treatment:
CDC avaccination against strains A, C, and Y.
Treatment includes spinal tap & immediate treatment of antimicrobial drugs has reduced mortality to less than 10%.
Prokaryotes; members of the intestinal microbiota of most animals and humans.
Most popular Gram-negative bacteria in humans.
Virulence factors of Enteric Bacteria:
-Capsules that protect the bacteria from phagocytosis and antibodies, and provide a pooerly immunogenic surface.
-Fimbriae and proteins called adhesions, which enable the bacteria to attach tightly to human cells.
-Extoxins that cause a variety of symptoms such as diarrhea. Genes for exotoxins, fimbriae, & adhesions are frequently located on plasmids, which increase the likelihood that they will be transferred among bacteria.
More virulence factors of Enteric Bacteria:
-Hemolysins, which release nutrients such as iron by lysing red blood cells.
-Lipid A, which, when released in a patients blood, can trigger fever, vasodialation, inflammation, shock and DIC. Because of Lipid A, most enteric bacteria can cause serious disease and death.
What is the difference between a "true" pathogen and other enteric pathogens?
"True" pathogens generally cause disease upon infection, while the others only occasionally cause disease.
Diseases caused by E. Coli O157:H7
UTI: More likely in women - ascends from bladder, infecting kidneys and causing a more serious disease - acute pyelonephritis, involves fever, flank pain, bacteria in urine, perspiration, nausea and vomitting.
Travelers Diarrhea: watery diarrhea
HUS: Hemolytic uremic syndrome, severe kidney disorder, can cease renal function.
Shiga-Toxin in E.Coli:
Produced by E.coli - inhibits protein synthesis in host cells. It attaches to the surfaces of neutrophils and is spread by them throughout the body and can cause widespread death of host cells and tissues.
Antimicrobial drugs induce E.Coli to increase its production of Shiga-like toxin, worsening the illness.
Salmonella typi (Gram-Negative bacilli):
-From animal feces, most likely pet reptiles (birds).
Typhoid Fever: Infection occurs via ingestion of food or water contaiminated with sewage containing bacteria from carriers. (Typhoid Mary).
High fever, significant mortality.
Treatment: Quinolones: Cephalosporins
-From consumption of inadequantly pasteurized contaiminated milk.
Nausea & diarrhea.
Treatment: Oral rehydration
-Passed from human to human
Severe form of dysentery called shigellosis, which is characterized by abdominal cramps, fever, diarrhea, and pus-containing bloody stools. Associated with poor personal hygeine.
Enteric pathogtion of food or water contains acquired via the consumption contaiminated by animal feces.
-Common cause of painful inflammation of the intestinal tract. Diarrhea and fever can last for months.
Yersinia pestis (nonenteric)
Extremely virulent, nonenteric pathogen that has 2 clinical manifestations: bubonic plague and pneumonic plague.
Not spread from person to person, rather from direct contact from a flea or insect that bites them.
Characterized by high fever and swollen, painful lymph nodes called buboes.
Black Death. Fatal in 50% of cases.
Occurs when Yersinia in the bloodstream infects the lungs.
Develops rapidly - develop fever, malaise, and pulmonary distress within a day of infection.
Can spread from person to person.
Bordetella pertussis (Gram negative coccobacillus)
-Pertussis toxin: a portion of which interferes with the ciliated epithelial cell's metabolism, resulting in increased mucus production. (Both adhesion and a toxin).
-Tracheal cytotoxin, which at low concentrations inhibits the movement of cilia on ciliated repiratory cells, at high concentrations causes the expulsion of the cells from the lining of the trachea.
P of DTaP and of Tdap - no animal or environment resevoirs - could be irradicated if vaccinated.
The presence of lipid A in the outer membranes of Gram-Negative bacteria ________.
Affects the formation of blood clots in the host
The only genus of Gram-negative Cocci that causes significant disease in humans is _____.
Capsules of pathogenic enteric bacteria are virulence factors because they _________.
Protect the bacteria from phagocytosis and from antibodies
Salmonella enterica (serotype Typhi)
Pus-filled, bloody stools; cramps; fever and diarrhea
-Enters through abrasions and lacerations and infects a limited array of cells.
-Most commonly reported STD.
-Can be transmitted from eye to eye via droplets, hands, contaminated fomites, or flies. Also can be transmitted during birth.
Lymphogranuloma Venereum (C. trachomatis)
A transient genital lesion and swollen, panfully inflamed, inguinal nodes (buboes).
-Caused by the so-called strain of LGV strain of C. trachomatis.
-Headache, muscle pain , and fever may occur at this stage of the disease.
Trachoma (C. trachomatis)
-Cause a disease of the eye called trachoma, which is the leading cause of nontraumatic blindness in humans.
-Can survive in freshwater, most likely species to infect humans.
-Cholera (particularly O1 El Tor) one of the worlds most pernicious diseases.
-Become infected by ingesting contaminated food/water.
Symptoms: Develop 2-3 days following infection; expolsive, watery diarrhea and vomitting.
Most important virulence factor
1. One of the B subunits binds to a glycolipid receptor in the cytoplasmic membrane of an intestinal epithelial cell.
2. The A subunit is cleaved, and a portion (called A1) enters the cell's cytosol.
3. A1 acts as an enzyme that activates adenylate cyclase (AC).
4. Activated AC enzymatically converts ATP into cyclic AMP (cAMP).
5. cAMPT stimulates the active secretion of excess amounts amounts of electrolytes (sodium, chlorine, tassium, and bicarbonate ions) from the cell).
6. Water follows the movement of electrolytes from the cell and into the intestinal lumen via osmosis.
Treatment for V. cholera:
Fluid & electrolyte replacement.
Doxycyline for adults and ampicillin for children.
Proper hygeine is best protection.
Slightly helical, colonizes the stomachs of its hosts.
Causes gastritis and most peptic ulcers, which are erosions of the mucous membrane of the stomach.
Significant risk for stomach cancer.
Portal of entry is the mouth, feces on hands, well water, or fomites may infect humans.
-Urease protects H. pylori.
Virulence factors H. pylori:
Protein that inhibtits acid production, urease, flagella, adhesions, antiphagocytic enzymes.
Treatment of H.pylori:
Antibacterial drugs in conjunction with drugs that inhibit acid production in the stomach.
Most common cause of bacterial gastroenteritis in the US.
They are zoonotic - resevoirs include poultry, dogs, cats, rabbits, pigs, cattle, and minks.
Acquire by consuming milk, food, water contaiminated with animal feces.
The smallest cellular microbes are _______.
Rickettsias and chlamydias
The most commonly reported STD in the US is caused by the bacterium ________________.
Treatment of chlamydial infections involves _________.
Erythromycin cream, doxycyclin creams, and surgical corrections of eyelid deformities.
What is not true of cholera?
There is an effective vaccine for cholera.
The most common cause of bacterial gastroenteritis in the US is _________________.
What is a virus? How is it different from other pathogens?
Miniscule, acellular, infectious agent usually having one or several pieces of nucleic acid - either DNA or RNA.
Lack cytosol and functional organelles. Not capable of metabolic activity on their own, thus taking control of a cell's metabolic machinery to produce more molecules of viral nucleic acid and viral proteins.
What a virus is called outside of a cell, in the extracellular state. Consists of a protein coat (capsid), surrounding a nucleic core.
Protein coat surrounding a virion.
A phospholipid membrane that surrounds the nucleocapsid (some virions have this).
Doublestranded DNA viral genome - in herpesvirus and chicken pox.
viral genome: single-stranded RNA.
Virus that infects bacteria.
They outnumber all bacteria, archaea, and eukarytes put together.
Dependent on their hosts enzymes and organelles to produce new virions.
The replication cycle usually results in the death and lysis of the host cell.
Lytic Replication Cycle
1. Attachment of the virion to the host cell
2. Entry of the virion to its genome into the host cell
3. Synthesis of new nucleic acids and viral proteins by the host cells enzymes and ribosomes.
4. Assembly of new virions within the host cell.
5. Release of the new virions within the host cell.
How does the lytic cycle differ from the lysogenic cycle in bacteriophage?
Infected host cells grow and reproduce normally for many generations before they lyse called lysogeny.
Difference between replication of animal viruses and bacteriophage replication:
Same 5 steps: however, there are differences. The presence of envelopes around some of the viruses and from the eukaryotic nature of animal cells and lack of a cell wall.
-Membrane fusion allows the virus to fuse envelope and membrane to enter the host cell.
-Most enveloped viruses enter host cell by triggering endocytosis.
-Enveloped animal viruses are often released via budding, which allows a host cell to remain alive during sometime (rather that lyse like the bacteriophage process) - called persistant infections.
Extrusion of enveloped virions throught host cell's membrane.
How is latency different from lysogeny? How is it the same
Differences include some latent viruses do not become incorporated into the chromosomes of their host cells, whereas lysogenic phages always do.
However, some animal viruses (HIV) are more like lysogeny in that they become integrated into a host chromosome as a provirus.
How do viruses contribute to cancer?
Revolve around the presence of protooncogenes- genes that play a role in cell division.
As long as they are repressed, no cancer results. However the activity of oncogenes (name when they are active) or inactivation of oncogene repressors can cause cancer to develop.
Viruses lack a cytoplasmic membrane.
A virus that is specific for a bacterial host is called a ________.
A naked virus _________.
Has no membranous envelope.
When a eukaryotic cell is infected with an enveloped virus and sheds viruses slowly over time, this infection is ______________.
called a persistent infection.
Another name for a complete virus is ___________.
A clear zone of phage infection in a bacterial lawn is _____________.
Dormant virus in a eukaryotic cell:
A virus that infects a bacterium:
Transcribes DNA from RNA:
Protein coat of virus:
A membrane on the outside of a virus:
Complete viral particle:
Inactive virus within a bacterial cell:
Removal of capsomeres from a virion:
Invasive neoplatic cells:
Harmless neoplastic cells:
(Both known as simplexvirus)
Known as above the waist herpes; first herpes virus to be discovered.
Produce painful, itchy skin lesions on the lips, fever blisters or cold sores. Flu-like signs/symptoms, mailase, fever, muscle pain.
80% of children have been asymptomatically infected.
Below the waist herpes; differs from HHV-1 in surface antigens, primary location of infection and mode of transmission.
Associated with painful lesions on genitalia, because its typically transmitted sexually.
HHV-3 (Varicella Zoster)
-Varicella- disease of children
-Herpes zoster - adults.
Varicella is chickenpox, enter through respiratory tract or eyes, travel throughout the blood, triggering fever, malaise, and skin lesions.
Zoster (or shingles) is a painful rash that stems from having chicken pox.
Chicken pox requires typically no treatment besides acetaminophen and antihistamines.
Shingles treated by bed rest and symptom management.
Associated with a variety of diseases; Burkeitt's lymphoma (seen in African boys, and first virus shown to be responsible for cancer) and infectious mononucleosis (mono; kissing disease).
Transmission through saliva and invades B lymphocytes.
Burkitts - chemo or surgerical removal of tumors.
-Cells infected with the virus become enlarged. One of the most common human infections.
Transmitted in bodily secretions, saliva, mucus, milk, urine, feces, semen, and cervical secretions.
Usually via sexual intercourse. Most people infected are asymptomatic. Can be teratogenic, cause birth defects, if it infects stem cells.
Treatment is difficult for newborns and adults alike; injections formivirsen into an infected eye.
No vaccine for preventing infection.
Abstinece reduces infection.
-Cause papillomas, benign growths of the peithelium of the skin or mucous membranes known as warts.
Genital worts are worse in that increase risk of cancer.
-Transmitted via direct contact and via fomites. Also autoinoculation.
-HPV virus targets and replicates in skin cells and genitalia.
-Certain strains can contain oncogenes linked to causing cervical cancer.
Virus remains asymptomatic in host cells for long periods of time.
Hepadnaviridae (Hep B virus)
Acute symptoms vary; from mild (appetite loss and low grade fever) to more severe (fever, jaundice, nausea).
-A fraction of those that recover remain persistantly infected. They serve as a resevoir and have a high incidence of liver disease.
-Transmitted by parental blood or sexual contact.
-Prevention Hep B vaccination or a recombinant subunit vaccine.
How does persistant infection infection differ from latent?
After acute infection subsides, a fraction of the hosts remain persistantly infected.
Whiich of the following viral families is most likely to contain viruses that exist in a latent state in humans?
Human herpesvirus 2 _____________.
can cause genital herpes, may infect a baby at birth, and causes about 10% of cold sores.
Epstein-Barr virus ____________.
can be asymptomatic, causes mono, and can cause cancer.
Flavivirus (enveloped, unsegmented -ssRNA virus)
Marburg virus and ebola are the causative agents.
-natural resevoir and mode of transmission to humans is unknown.
-Spread person to person by bodily fluids and syringes.
-Virions attack macrophages and liver cells
-Uncontrolled bleeding under the skin at every body opening
-Viral glycoprotein prevents neighboring cells from adhering (allows blood to leak out of vessels)
-UP to 90% of human victims die; symptoms are diagnostic
-Only treatment is fluid replacement
Dengue Fever Virus (Breakbone fever) (+ssRNA virus)
-Aedeos mosquitoes transmit this flavivirus in tropical/subtropic areas.
-2 phases: A week of high fever, weakness, edema of extremedies, severe pain. Remission for 24 hours, then return of fever and bright red rash.
-Hemorrhagic fever is more serious disease caused by reinfection, involved hyperimmune response.
-No treatment, experimental vaccine that is promising.
-Mosquito control can limit disease.
Influenza (flu) (Enveloped, segmented -ssRNA)
Symptoms: Chills, fever, headache, & muscle aches. (NO intestinal symptoms)
-1% mortality rate, very young & very old
Main Antigens of Influenza
-Hemaggluginin: (HA) spikes used for attachment to host cells
-Neuraminidase: (NA) spikes used to release virus from cells.
Antiviral therapy. What makes good therapy? Why is reverse transcriptase good?
The problem is that viruses use host cells to replicate, so many treatments that we can use to fight bacteria just aren't relevant because they would damage our cells. So, we need to find unique features of the virus to target. Common things are attachment sites (like NA/HA) because these viral proteins are only found in viruses. The other things are viral specific enzymes.
Reverse transcriptase inhibitor is a good example of this last category. Generally human cells do not need reverse transcriptase to replicate, however HIV
needs reverse transcriptase. So, if we block reverse transcriptase with a reverse transcriptase inhibitor we block HIV from replicating and inserting into the host cell DNA, but we don't damage human cells
Antigenic Shift vs Antigenic Drift
Shift: Changes HA and NA spikes; probably due to genetic recombination between different strains infecting the same cell.
Drift: Point mutations in genes encoding HA or NA spikes; may involve 1 amino acid. Allows virus to avoid mucosal IgA antibodies.
Surveillance: monitoring of the most prevalent flu strains of that year.
-Most prevalent strains are used to make either whole-aged killed vaccines or attenuated live vaccines.
-Treatment: Zanamivir and oseltamivir.
Antiviral drugs that inhibit neuraminidase activity.
HIV (enveloped +ssRNA virus)
Caused by AIDS.
-Virulence factors: attaches to CD4 of helper T, vigorous mutation rate due to transcription errors of reverse transcriptase, latency.
Primary attachment molecule of HIV to enter target cells.
Reverse Transcriptase in HIV
Carried inside the capsid, becomes active in the cytosol.
RT is very error prone therefore generates multiple antigenic variations of HIV.
HIV is a latent virus
dsDNA made by revers transcriptase is known as a provirus, and enters the nucleus. May remain dormant for years or be activated immediately, depending on its location in the human genome and the availability of promoter DNA sequences.
drugs that interfere with the function of protease - have become a standard therapuetic agent in the treatment of HIV infection and significantly lengthened the life expectancy of patients.
Antiretroviral Therapy (ART)
-Reverser transcripase inhibitors (Nucleoside analogues and non-nucleoside inhibitor)
Highly active antiretroviral therapy (HAART)
Combo of multiple antiretroviral drugs (nucleoside analogues as well as non-nucleoside or RT-unrealated inhibitor)
Arboviruses are _________.
viruses that are transmitted to humans via the bite of an arthropod.
A horror movie portrays victims of biological warfare with uncontrolled bleeding from the eyes, mouth, nose, ears, and anus. What actual virus causes these symptoms?
Serves as a host for Salmonella enterica serovar typhi?
Because Campylobacter species have an animal reservoir, diseases caused by these organims are considered ______ disease.
Helicobacter is a(n) __________.
what does the incidence of gonorrhea in 2010 mean?
Number of cases of gonorrhea diagnosed in 2010.
IgA is most commonly associated with _________.
Neisseria gonorrhoeae is able to withstand immune system by all of the following means EXCEPT _______.
-Inactivation of complement
-Survival in phagocytes
-Variation of surface antigens
Inactivation of complement.
The formation of a new virus in the pig is an example of:
CD4 is predominantly found on ______.
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