All humans have friendly bacteria living in their intestines producing vitamins which they absorb.
These friendly bacteria also help to keep the growth of harmful yeasts under control. Although the bacteria can directly attack other microorganisms, friendly bacteria may also help the immune system. It is known that as humans age the effectiveness of the immune system declines. In people with HIV infection, ageing is another factor which may make older people less resistant to the effects of HIV infection. Researchers in Italy have been conducting experiments on elderly humans using friendly bacteria.