Read the following passage focusing your attention on the scientific procedures and scientific abilities illustrated in it. Think about what types of experiments (observation, testing or application) are described. Identify the hypothesis, predictions, outcomes of experiments and assumptions that appear in the passage. The standard explanation of how humans originally acquired AIDS is that HIV (human immunodeficiency virus) originally was transmitted to humans from monkeys or apes carrying SIV, simian immunodeficiency virus. How could the transmission have occurred? Several similar hypotheses involve an African hunter coming in contact with monkey blood. For example this could have occurred when a hunter butchering a monkey, got some of its blood in a cut, or when a human ate undercooked monkey meat, or when a monkey bit a human. Depending on one's assessment of the genetic diversity of HIV over time, this transmission might have occurred only a few decades ago or, alternatively, hundreds or thousands of years ago. If the latter is correct then AIDS could have been restricted to remote African villages until recent decades when trade and international travel led to the current pandemic. There are however other alternative explanations for the spread of AIDS. One of them is that AIDS arose from contaminated polio vaccines used in Africa from 1957-59. Why is this a plausible explanation? Polio vaccines are cultured on apes’ kidneys, so contamination of a batch of vaccine by an SIV is a possibility. There is a precedent: in the 1950s and early 1960s millions of doses of polio vaccine were contaminated by another simian virus, SV-40. The vaccine alleged to be the origin of AIDS was given to hundreds of thousands of people in central and west Africa from 1957-1959. These locations later had some of the world's highest levels of HIV infection. The timing is also right, since the earliest known cases of AIDS date from the 1960s; the earliest HIV-positive blood sample was collected in Kinshasa in 1959. At the time, there was no way of testing for the presence of SIVs, which were only discovered in the 1980s. Another factor adds plausibility to the theory: the vaccine was given to many infants less than a month old, in an extra high dose. Since the immune systems of infants are not fully developed, this is an excellent way to help a virus jump the species barrier. Some scientists argue that the above explanation is not necessarily correct but that it is worth taking seriously. There are a number of ways to test it. One is to analyze stocks of polio vaccine; if the explanation is correct, then it is probable that some of the vaccines will carry SIV. Another is to scrutinize records of early polio vaccination campaigns in Africa. If it is true that the immunodeficiency virus passed to humans by way of those vaccines, the areas where the campaigns took place would be the same as those where the initial sprouts of the disease occurred and where the larger number of cases occurred during the early stages of the pandemic. Another is to analyze blood samples in Africa from the 1950s and earlier (pre polio vaccination): if the immunodeficiency virus was transmitted via the polio vaccine, HIV should not be found. Adapted from “Political refutation of a scientific theory: the case of polio vaccines and the origin of AIDS”. Brian Martin, published in Health Care Analysis, Vol. 6, 1998, pp. 175-179. Now reread the passage and answer the following questions.
5. For each experiment, write an outcome that would disprove the hypothesis of AIDS passing from monkeys to humans through polio vaccines.
6. What outcomes of the experiments would fail to disprove the hypothesis of AIDS passing from monkeys to humans through polio vaccines?explain.
7.What outcomes would prove the hypothesis? Hint: This is not the same as question 6. Think about what would you need to do to 100% prove a hypothesis.
8. For what reasons do scientists need to conduct testing experiments?
