Three to six weeks after exposure to the virus, up to 70 percent of HIV-infected individuals suffer flu-like symptoms related to the acute infection, such as fever, malaise, sore throat, headaches and swollen lymph nodes.
After several weeks, the patient's immune system fights back. B cells produce antibodies that neutralize some of the free virus and killer T cells destroy many HIV-infected cells. Then, the individual usually goes into a symptomless stage of infection that lasts an average of 10 years. The virus is not totally latent, but replication is slow.
The immune system deteriorates slowly during this period as the number of crucial immune cells in the bloodstream, known as CD4+ T cells, gradually declines.
Ultimately, HIV overwhelms the lymphoid organs. Large quantities of virus spill into the bloodstream and the body cannot mount a successful immune response against the HIV or other pathogens. This allows the onset of opportunistic infections and cancers that characterize AIDS. In 1993, the U.S. Centers for Disease Control in Atlanta changed its definition of AIDS to situations in which the T helper cell count is less than 200. The entire process is more rapid in children than in adults. Interestingly, one-third of the children born to HIV-infected women are estimated to be infected, while two-thirds, though initially HIV-positive because of passively acquired maternal antibodies, will lose antibodies by 15 months and prove to be uninfected. Then, however, they can become infected by breastfeeding.