It spreads mainly through people having sexual relations but women with HIV can give it to babies during pregnancy and birth and through breastfeeding them. The virus attacks the body's natural defences - the immune system. People can have HIV for several years without knowing it, but in many cases eventually their protection cells stop working and they develop other illnesses which are fatal. From the time someone is infected with HIV it is possible to pass it on to somebody else. One of the many reasons why Aids has hit developing countries so hard is because often they do not have the equipment or systems needed to test people for the disease.
People remain unaware that they are infectious and so put others they have sex with at risk.
No one knows. It is similar to viruses that affect monkeys and apes. One idea is that HIV first got into humans in Africa when someone killed an infected chimpanzee and its blood went into an open wound on the hunter's body. It has also been linked to a Polio vaccine that was developed using chimpanzees. There are drugs that can delay HIV developing into Aids and there are others that can stop the virus passing from mothers to their children in breast milk.