Question: Where did HIV , cancer and TB originally come from?

Susan Cartwright answered on 16 Jun 2015:

HIV is a cross-infection of humans by a virus that normally infects apes and monkeys (in most of which it does not cause severe disease). Humans probably acquired it by eating the apes and monkeys. Genetic studies suggest that the two strains of HIV, HIV-1 and HIV-2, derive respectively from chimpanzees and sooty mangabey monkeys.

The origins of TB are disputed, but some recent genetic studies suggest it is an old human disease which originated in humans about 70000 years ago. Its spread has been helped by cross-infections between humans and domestic animals, especially cattle and goats: it seems to have done rather well out of the development of farming.

Cancer is not one disease, but a whole family of them. Some appear to be side-effects of viruses – that’s why these days you can be vaccinated against cervical cancer. Most of them are caused by mutations in genes, which cause the mutant cell to multiply out of control. It’s probably a feature of all multicellular organisms – even plants get something related to cancer, although it is much less dangerous to them because it cannot spread easily (plant cells have rigid cell walls, and plants do not have circulating blood). So cancer has always been with us, although some features of modern life (cigarettes, asbestos, etc.) make some types of cancer more common, and the fact that we are otherwise living longer also makes it more likely – the longer you live, the more time there is for some accidental cancer-causing mutation to happen in one of your cells.