HIV and AIDS. HIV (Human Immunodeficiency Virus) infection is a slowly progressive viral disease of the immune system that leads to a weakening of the immune defense against tumors and other infections. Acquired Immune Deficiency Syndrome (AIDS) is the stage of HIV infection during which secondary infections or tumor diseases appear due to the declining immunity. AIDS spreads in geometric progression and the vaccine against it is still not found. The only way to restrict its spread is to teach people how to avoid contract with a deadly virus. HIV is transmitted through blood, sperm, vaginal secretions and breast milk. There is no risk of getting the infection through other biological materials like saliva, sweat, tears, urine and feces. This happens because a certain minimal concentration of the virus is required to get infected. The amount of virus required for infection may be found in a blood drop that locates on the end of the sewing needle, while the volume of saliva which contains the same amount of virus is 4 liters. The immune system weakens gradually. A person with HIV may look and feel well for many years and did not even know he is infected. However, the virus destroys more and more cells of the immune system over time. When the number of cells reduces below a critical level, a person becomes susceptible to diseases, including those to which people with normal immunity are resistant. The period after infection and before the appearance of HIV antibodies in the blood is called a “window period.” It may last from 25 days to 3 months after the very infection. Using a special kind of blood analysis it is possible to detect antibodies to the virus. A blood test for HIV can be made in any hospital.