Viruses are the simplest life forms. Many diseases, such as measles, mumps, the cold and flu are caused by viruses. A typical virus shows a central or core portion made of DNA (Deoxyribonucleic acid). This establishes the genes of a virus-chemical codes needed to build more viruses identical with it. The DNA core is covered by a sheath like a mosaic, composed of various protein molecules. However, a virus cannot replicate on its own, it needs a living cell, such as a cell in the body, to act as a host. The virus enters the host cell, the host adds its genes and determines the chemical mechanism of the cell to produce hundreds of thousands of copies of itself. Then these viruses exit the host cell, destroying it and are ready to infect other cells. HIV is an unusual virus, belonging to a group of viruses called retrovirus. They do not contain DNA, but RNA instead. Inside the host cell, the RNA is first converted into DNA, then the DNA is used as code for virus multiplication. HIV is unusual because its main effects are not felt for some time, generally ranging between 5-10 years after it entered the body. When a person is infected with HIV, the virus multiplies rapidly. It can be detected in both blood and cerebrospinal fluid. The person may not be affected at this stage and may have some flu-like symptoms such as runny nose and fever, rash, axillary lymphadenopathy or frequent headaches. These symptoms are often flu-like. These symptoms and high levels of HIV in the body usually go away after a few weeks, when the infected person feels fine again. The virus is present but inactive. Unconsciously, the person can transmit the virus. Eventually, after many years, HIV is reactivated and starts to multiply again. That is the time when AIDS occurs. Oral thrush and leukoplakia hairy often appear. Oral thrush appears as white spots on the tongue, leukoplakia consists of some white bumps along the tongue. At this stage, the immune system begins to fight, and the number of white blood cells decreases. In some cases or in others, gradually after a few weeks, begin to develop various diseases. They are skin and mucous membranes infections. The body may yield to some infections, such as pneumonia and tuberculosis or meningitis or encephalitis. Such inflammation can cause confusion or mental problems (including dementia). Also the following can occur: blurred vision, diarrhea or digestive problems with excessive bleeding and wounds. Other conditions that may occur are different forms of cancer, Kaposi’s sarcoma, which causes skin lesions. After a few weeks or months, the patient enters the last stages of the disease, as various diseases overwhelm the body.