For example, testing the susceptibility of different strains of rats to infections can help scientists discover why some human beings fall prey to infections and diseases, while others do not catch the disease at all. At the same time, cells of the same type, even though from different species, can behave in almost identical ways. The strength of using animals in research lies in knowing both the similarities and differences between animals and humans. Vaccines developed using animal research are unsafe. Smallpox vaccine, for example, is said to have caused severe illness and even death in some people.
Smallpox vaccine is one of the most successful preventative medicines ever used.
It has completely eradicated this disfiguring and potentially fatal disease from the world. Doctors were all aware of the occasional severe side effects of smallpox vaccine. That is why, once a country had finally eradicated the disease through use of the vaccine, there was debate in medical journals over whether to discontinue use of the vaccine. While a superficial look at the debate makes it appear many doctors were opposed to vaccination, this was not the case. Ironically, the very success of vaccination has tended to encourage a casual attitude towards these diseases.