Parallels between animal and human diseases. Some infectious diseases only occur in particular animal species and cannot be caught by others, but the infectious agent (e.g. a bacterium or a virus) may belong to a family where different members of it do cause similar diseases in a different animal species (e.g. the viruses that cause AIDS in people and a similar disease in cats). Comparing the way the bodies of the different species respond to their similar, but distinct, disease agents can improve our knowledge of the way the disease operates in both species.