If untreated or ineffectively treated, people with hiv/aids become increasingly vulnerable to diseases.

HIV is a retrovirus, a virus that uses RNA as its genetic material instead of the more usual DNA. Retroviruses infect cells by copying their RNA into the DNA of the cell, penetrating the genetic material. This DNA may sit silently in the cell, being copied normally along with the cell's genetic material for an indefinite number of generations. Or it may take over control of the cell's machinery, causing the uncontrolled reproduction typical of cancer. The target of HIV is a specific type of white blood cell called the CD4-T lymphocyte, or T4 cell. The T4 cell's role is to divide and reproduce itself in response to such an invasion and to attack the invader. In a T4 cell that is infected with HIV, activation of the cell activates the virus also, which then produces thousands of copies of itself in a process that kills the T4 cell. The T4 cells are a key component of the immune system. The course of infection with HIV takes place over a number of years. After being exposed to HIV, a person may or may not notice mild, flu-like symptoms for a few weeks, during which time the virus is present in the blood and body fluids and may be easily transmitted to others by sex or other risky behaviors. The body's immune system responds as it would to any viral infection, producing specific antibodies that eliminate most of the circulating viruses. The infection then enter a latent period, with the viruses mostly hidden in the DNA of the T4 cells, although a constant battle is taking place between the virus and the immune system. Billions of viruses are made, and millions of T4 cells are destroyed daily. During this time, the person is quite healthy and is less likely to transmit the virus than during the early stage of infection. Eventually however, after several years, the immune system begins to lose the struggle. When the number of T4 cells drops below 200 per cubic millimeter of blood, symptoms are likely to begin appearing, and the person is vulnerable to opportunistic infections and certain tumors. At the same time, the number of circulating viruses increases, and the person again becomes more capable of transmitting the infection to others. At this stage, the person meets the criteria for AIDS, which is defined by the T4 cell count and/or the presence of opportunistic infections.

Oxygen and glucose are both reactants in the process of cellular respiration. The main product of cellular respiration is ATP; waste products include carbon dioxide and water.

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