Like many other diseases, such as tuberculosis and malaria that cause illness and death in underprivileged and impoverished communities, AIDS spreads by infection. HIV-1, the retrovirus that is responsible for the AIDS pandemic, is closely related to a simian immunodeficiency virus (SIV) which infects chimpanzees. HIV-2, which is prevalent in West Africa and has spread to Europe and India, is almost indistinguishable from an SIV that infects sooty mangabey monkeys. Although HIV-1 and HIV-2 first arose as infections transmitted from animals to humans, or zoonoses, both are now spread among humans through sexual contact, from mother to infant and via contaminated blood.
An animal source for a new infection is not unique to HIV. The plague came from rodents. Influenza and the new Nipah virus in South-East Asia reached humans via pigs. Variant Creutzfeldt-Jakob disease in the United Kingdom came from 'mad cows'. Once HIV became established in humans, it soon followed human habits and movements. Like other viruses, HIV recognizes no social, political or geographic boundaries.
The evidence that AIDS is caused by HIV-1 or HIV-2 is clear-cut, exhaustive and unambiguous. This evidence meets the highest standards of science. The data fulfill exactly the same criteria as for other viral diseases, such as poliomyelitis, measles and smallpox:
- Patients with acquired immune deficiency syndrome, regardless of where they live, are infected with HIV.
- If not treated, most people with HIV infection show signs of AIDS within 5-10 years.
- HIV infection is identified in blood by detecting antibodies, gene sequences or viral isolation. These tests are as reliable as any used for detecting other virus infections.
- Persons who received HIV-contaminated blood or blood products develop AIDS, whereas those who received untainted or screened blood do not.
- Most children who develop AIDS are born to HIV-infected mothers. The higher the viral load in the mother the greater the risk of the child becoming infected.
- In the laboratory HIV infects the exact type of white blood cell (CD4 lymphocytes) that becomes depleted in persons with AIDS.
- Drugs that block HIV replication in the test tube also reduce viral load and delay progression to AIDS. Where available, treatment has reduced AIDS mortality by more than 80 percent.
- Monkeys inoculated with cloned SIV DNA become infected and develop AIDS.