The British epidemiologists Richard Doll and A. Bradford Hill sent out a questionnaire to all physicians in the UK, asking whether they were smokers, past smokers, or nonsmokers. Smokers and ex-smokers were asked to provide additional information on their age at starting to smoke and the amount of tobacco smoked, and ex-smokers were asked when they had quit smoking. During the following years, Doll and his collaborators gathered information on which doctors had died each year and what was the cause of death. A little over 4 years after the survey began, several important conclusions were apparent: First, the death rate from lung cancer was about 20 times higher among smokers than among nonsmokers, increasing as the amount smoked increased. Second, the death rate among ex-smokers was lower than that of smokers and declined as the length of time increased since the doctor had quit smoking. Third, the contrast in lung cancer mortality between smokers and nonsmokers was the same whether the doctors lived in rural or urban areas. Therefore, the difference could not be attributed to air pollution. Fourth, deaths from heart attacks were also significantly higher among heavy smokers aged 35 to 54 than among nonsmokers. In 1948, an epidemiologic study was launched in Framingham, Massachusetts, to investigate factors that might be causing the problem. It was the first major epidemiologic study of a chronic disease. More than half of the middle-aged population of the town, more than 5000 health people, were examined, and data were recorded on their weight, blood pressure, smoking habits, the results of various blood tests, and other characteristics. Two years later, the same people were examined again, and these tests have been and continue to be repeated every 2 years for the rest of their lives. As early as 10 years later, the Framingham Heart Study had revealed a great deal about how to predict which of their subjects were likely to develop heart disease. The study identified three major risk factors: high blood pressure, high blood cholesterol, and smoking. As a result of these findings, concepts of normal blood pressure and cholesterol levels changed significantly. Doctors had previously believed that blood pressure naturally increased as people aged and that the increase was normal and healthy. Remarkably, the Framingham findings had a major impact on the course of the heart disease epidemic. Publicity on the information gained by the study, confirmed and supported by other studies, persuaded some people to change their behavior and formed the basis of public health programs to encourage others to do the same. By the 1970s, it was clear that death rates from heart disease were falling in the US. These beneficial trends have continued. Birth certificates contain information supplied by the mother about the child's family, including names, addresses, ages, race and ethnicity, and education levels. Medical and health information is supplied by the hospital, doctor or other birth attendant concerning prenatal care, birth weight, medical risk factors, complications of labor and delivery, obstetrical procedures, and abnormalities in the newborn. While bacteria are living, single-celled organisms can grow and reproduce outside the body if given the appropriate nutrients. Viruses are not complete cells, they are simply complexes of nucleic acid and protein that lack the machinery to reproduce themselves. Various kinds of viruses infect not only animal cells but also plant cells and even bacteria. They can survive extreme conditions. They reproduce themselves by taking control of the cell's machinery, often killing the cell in the process. The human diseases caused by viruses include smallpox, yellow fever, polio, hepatitis, influenza, measles, rabies, and AIDS, as well as the common cold.