Human immunodeficiency virus (HIV) is an example of a virus that spreads from other animals to humans where it causes disease. [Figure 2.1] shows a chimpanzee, *Pan troglodytes troglodytes*. Chimpanzees can carry the simian immunodeficiency virus (SIV) which is similar to HIV. It is thought that chimpanzees who carry antibodies for SIV do not become ill if infected with HIV. This has been investigated by scientists developing potential vaccines for HIV. Tests were carried out to see if antibodies against SIV present in chimpanzees bind to HIV antigens. Test strips were prepared which contained several different HIV antigens. When samples are applied to the test strip a line will appear in the control region. If the sample contains antibodies to the HIV antigens present on the strip, additional lines will also appear. Samples of chimpanzee faeces were collected from a number of sites in Gabon in central Africa. The faecal samples were prepared and then applied to the test strips.