How does HIV progression into AIDS work?

HIV infection attacks white blood cells known as CD4 lymphocytes, which coordinate the efforts of the immune system in attacking disease. Normally, white blood cells and antibodies attack and destroy foreign organisms, know as antigens, in the body. HIV attaches to CD4 lymphocytes and "hijacks" them by inserting its own genetic material into the cells and replicating itself.

Eventually, these copies of the virus enter the bloodstream and begin attacking other cells. When this occurs, the original CD4 lymphocyte that spawned the copy dies. The cycle repeats itself over and over, with more than 10 billion new HIV particles produced every day. The immune system reacts by trying to create vast quantities of new CD4 cells – as many as 2 billion new cells each day - but eventually finds itself overpowered by the virus.

As the virus continues to overwhelm the immune system, the number of CD4 cells in the body decreases. This results in severe immune deficiency, in which the body is left unable to fight off viruses and bacteria. The deficiency then becomes AIDS when the CD4 count is less than 200.

What are opportunistic infections and what do they have to do with HIV or AIDS?

Opportunistic infections are a result of the damage that HIV inflicts upon your immune system. A few examples of opportunistic infections include:

• Bacterial pneumonia. Among the most common infections occurring in people living with HIV and AIDS, it can be caused by dozens of types of bacteria. Bacterial pneumonia develops on its own or after a person has had an upper respiratory infection such as a cold or the flu.
  
  
• Mycobacterium avium complex. Often abbreviated as MAC, this infection is caused by a group of bacteria collectively known as mycobacterium avium complex. In many cases, MAC causes an infection of the respiratory tract. But people with advanced HIV infection and CD4 lymphocyte counts of less than 50 may develop a systemic infection of almost any internal organ, including the bone marrow, liver or spleen. Symptoms of MAC include night sweats, weight loss, stomach pain and diarrhea.
  
  
• Tuberculosis. Also known as TB, this infection is the leading cause of death among people living with AIDS in developing countries. People with HIV are more susceptible to TB, and TB increases the rate of AIDS virus replication.
  
  
• Human papillomavirus. Also known as HPV, this is among the most common causes of sexually transmitted diseases. HPV can cause genital warts and other warts. People with HIV are more susceptible to infection with HPV and are more prone to recurrent infections. In women, HPV infection increases a woman's risk of cervical cancer. Infection with HPV and HIV further increases a woman's risk of aggressive cervical cancer. About half of all women with HPV have no symptoms.
  
  
• Cytomegalovirus. This is a common virus transmitted in body fluids such as saliva, blood, urine, semen and breast milk, and from mother to fetus during pregnancy. A healthy immune system inactivates the virus, and it remains dormant in the body. But diseases such as HIV that weaken the immune system can activate the virus, causing damage to the eyes, digestive tract, lungs or other organs.
  
  
• Cryptococcal meningitis. Meningitis is an inflammation of the membranes and fluid surrounding the brain and spinal cord, or meninges. Cryptococcal meningitis is caused by a fungus found in soil and is the most common central nervous system infection associated with HIV. It is rarely seen in non-HIV patients in the United States.
  
  
• Pneumocystis carinii pneumonia. This is the most common opportunistic infection that occurs with AIDS in the United States. It attacks the lungs and makes it difficult to breathe. Symptoms include a persistent cough, fever and trouble breathing.
  
  
• Toxoplasmosis. The most common cause of brain lesions in people with advanced HIV, this is caused by Toxoplasma gondii. House cats are primarily responsible for spreading this parasite, which passes through their stool. Women who are infected with the parasite can pass the infection on to their babies. Toxoplasmosis often leads to encephalitis, an infection of the brain. Signs and symptoms may include disorientation, seizures and difficulty walking or speaking.