| Term | Definition |
| parasite | an organism that lives in or on the body of another organism |
| ectoparasites | external parasites that live on their host but do not enter the host's body |
| endoparasites | internal parasites that live inside the host's body |
| pathogen | an agent that causes disease. |
| vector | a living organism that carries a disease-causing organism to new hosts |
| definitive host | the host that parasites go through their sexual stage with (this is the host where the parasites reproduce within) |
| intermediate host | a host that harbors the parasite but is not associated with sexual reproduction |
| reservoir host | the host that a parasite normally resides in |
| step 1 to success | reproduce within the definitive host |
| step 2 to success | get fertilized eggs or embryos out of the host |
| step 3 to success | contact and recognize a new, appropriate host |
| step 4 to success | obtain entrane into the host |
| step 5 to success | locate the appropriate environment within the host |
| step 6 to success | maintain position within the host |
| step 7 to success | withstand what is often a rather anaerobic (oxygen-poor) environment |
| step 8 to success | avoid digestion or attack by the host's immune system |
| step 9 to success | avoid killing the host, at least until reproduction has been achieved |