HIV can be spread from person to person, but only by a few means. The most common of these are: exchanging body fluids during sexual activity (semen, vaginal secretions, blood) and sharing used needles. Less commonly, HIV may be transmitted from mother to child during delivery or through breast-feeding. HIV has also been found in blood or blood products used in transfusions, although this type of transmission is now extremely rare due to sophisticated testing of all donated blood in the United States.
Just as important are the ways HIV is not transmitted. It is not transmitted by casual contact, hugging, using toilets after someone, sharing silverware or dishes, sweat, tears, urine, vomit, or saliva (unless there is blood).
Once HIV enters a new host (a person), it seeks out target cells to infect and hijack. The most commonly recognized of these target cells are the "T-cells" (otherwise known as CD4+ cells). In reality, HIV targets many different cell types within each host (not just T-cells), but they are most relevant for the health of your immune system.
The T-cells act like the generals of your own personal army. They don't do much fighting but they play a crucial role. They tell the rest of the army (your soldiers, captains, lieutenants, etc.) how to keep you safe from the enemy (germs). Without enough of these generals, your army becomes confused and isn't able to effectively fight infections. That's why as your T-cell count declines you become at greater and greater risk of catching a serious illness.
When HIV infects a T-cell, it doesn't kill it. Instead, it hijacks the cell, turning it into a factory to make more HIV. In fact, it's in the best interest of the virus to keep the T-cell alive as long as possible. The reason your T-cell count declines over time is because your immune system is able to recognize which T-cells have been hijacked and it tries to kill those cells off. Your body knows how many T-cells it's supposed to have, so it builds new ones to replace the ones it had to kill. However, as long as there is HIV present in the blood, sooner or later those newly created T-cells will also become infected with HIV.
Over time, more T-cells are being killed off than your body can replace and thus the T-cell count gradually goes down. A healthy adult should have (on average) between 500 and 1,200 T-cells. People infected with HIV will have anywhere from the normal range all the way down to zero. Most HIV-positive people do not come down with HIV-related problems until the T-cell count approaches and goes below 200. That is why many people with HIV don't know they have it at first. It usually takes several years after infection for one's T-cell count to drop from normal down to the risky zone below 200.
HIV is a global epidemic. Worldwide it is estimated that there are between 40 and 45 million HIV-positive people. The hardest hit areas are in Sub-Saharan Africa, India, and much of Southeast Asia. In those areas, men and women are nearly equally affected because the primary route of transmission is heterosexual contact.
In North America, there are estimated to be between 1 and 2 million HIV-positive men and women (most of them in the United States). Between 60-70% of these people are men, but the percentage of new infections in women (especially women of color, including Latinas) is increasing rapidly.