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C h e c k o u t A B M P 's l a t e s t n e w s a n d b l o g p o s t s . Av a i l a b l e a t w w w. a b m p . c o m . 43 percent of all diagnoses in the United States. Gay and bisexual men who are also racial minorities are particularly at risk: diagnoses among African Americans in this group have leveled off, but among Latinos, rates have risen. Heterosexual contact is a factor in about 24 percent of new HIV diagnoses each year, and injected drug use accounts for 6–9 percent of new diagnoses. HOW DOES HIV PROGRESS? HIV enters the body by way of an intimate fluid: blood, semen, breast milk, or vaginal secretions. This usually happens in the context of sexual activity, mother-to- child transmission, or through the use of contaminated needles or other skin-piercing equipment. Another avenue that was once a significant problem was the transmission of HIV through infected donated blood, blood products, or organ tissue. Strict screening now prevents this transmission route in the United States and most other countries. If the virus gains access through sexual activity, it bonds with epithelial cells in the genitourinary tract or the digestive tract. If it enters through maternal infection or the use of a contaminated needle or other implement, it is loose in the interstitial fluid or in the bloodstream. Either way, the virus then looks for cells with a particular marker on their cell membrane. This marker is a glycoprotein called CD4, and cells that have these markers are called CD4 cells. Here's the tricky part: most CD4 cells work for the immune system. This means that while HIV is looking for them, the CD4 cells are looking for any foreign material—like stray viruses. Monocytes and macrophages, the nonspecific white blood cells that constantly scan the body for invaders, are exactly the cells HIV is hoping to encounter, and they have the CD4 marker. When a monocyte picks up the invader, where does that monocyte go? To the nearest lymph nodes, of course—which is just what the HIV pathogen wants. Lymph nodes are home to many, many CD4 cells. Within the lymph nodes, the virus can invade latent T cells and stay there, literally, for years, only to become active when those T cells are recruited to help fight off an infection. Attentive readers might wonder: why doesn't the body raise the alarm when its cells have been invaded? One of the things that makes HIV so horribly successful is that it manages to interfere with its host cell's alarm system, at least for a while. So, it can enter its target cells—monocytes, macrophages, T cells, and others—and begin replicating and spreading through the body without creating an immediate immune system response. Hopeful Numbers It is interesting to track worldwide and US statistics on HIV and AIDS over the last several years. Most numbers are stable, or trending generally downward. But one statistic is radically different: the number of new infections in children is a fraction of what it was just 10 years ago. This is probably due to aggressive public health campaigns and successful efforts to ensure that pregnant women have access to medication that reduces the risk of passing the virus to their babies. The immune system does eventually wise up and fight back, but the individual loses valuable weeks or months while the virus is able to invade cells, damage tissues, and spread to other people before they know anything might be amiss. This also means that a blood test to look for antibodies to HIV may be inaccurate for up to six months after exposure and infection. HIV has two mechanisms for moving through a body. It invades its host cells and turns them into virus factories, so those cells now churn out copies and copies of HIV, or it can move directly from one CD4 cell to another, destroying the cells along the way. This happens in lymph nodes where lymphocytes congregate, and also among the microglial cells, which are the resident macrophages in the central nervous system. PROGRESSION AND SYMPTOMS So far, we have looked at how HIV enters a host and moves through the body. Now we'll look at the phases of HIV infection. Phase 1 Phase 1 of an HIV infection is the time between when the virus enters the body and when the immune system finally realizes that something is wrong and begins to fight back. This is sometimes called the window period. During this time, the virus is accumulating in the white blood cells, but it suppresses immune system responses. This means no antibodies are in production yet, so blood tests are negative, and no