From Epidemic to Pandemic
AIDS, acquired immune deficiency syndrome, is an infectious disease caused by the human immunodeficiency virus. AIDS is characterized by the appearance of opportunistic infections such as tuberculosis, fungal infections, meningitis, and syphilis. Weight loss and wasting are also effects of AIDS. The virus attacks immune system cells, and T cell counts and viral load counts are also important in both diagnosis and determination of appropriate treatment.
There is no cure for AIDS, but there are drugs that can treat the symptoms and infections that go along with the disease.
As the AIDS epidemic grew unabated in the United States, numerous cases appeared in Europe, South America, and Africa. While disease transmission was generally associated with gay men in the United States, disease transmission in other parts of the world was linked to heterosexual contact. In 1990, the World Health Organization (WHO) estimated that about one million people were living with AIDS. In less than 10 years, HIV had exploded worldwide.
In the early and mid-1990s, extensive education programs in North America and Europe, stressing the need for protection from bodily fluid transfer, helped to slow the spread of AIDS. Yet, around the world, the number of AIDS cases continued to rise. According to WHO, in 2001 an estimated 40 million people, 37.2 million adults and 2.7 million children, were living with HIV. Twenty years after the first cases were reported, a modern pandemic is ravaging the world's poor.
Most striking are the 28.1 million AIDS cases in Africa, with most in sub-Saharan Africa. Nearly 2.3 million Africans died from AIDS in 2001, and without adequate treatment and care, many more will die. Other countries, like India, are experiencing meteoric rises in AIDS cases. Some countries with little history of AIDS, like China and Russia, are poised for a major epidemic as well.
Clinical Definition of AIDS
The, CDC defines AIDS in an adult or adolescent aged 13 years or older as the presence of one of 25 conditions that indicate that the immune system is severely compromised because of HIV infection. People with HIV are also defined as having AIDS once their CD4 + T cell count goes below 200 cells per cubic milliliter of blood (normal CD4 counts are between 500 and 1600 per cubic milliliter).
AIDS refers only to the end stage of a progressive disease that starts with HIV infection and leads to severe impairment of the immune system. Once the immune system is weakened, AIDS patients are susceptible to other infections and cancers. These infections are called opportunistic because the organisms that cause them take advantage of the person's weakened immune system and cause disease.
T cell count is considered a critical measure of health because it tells whether the immune system is working properly. When our T cell count goes down, so does our ability to fight infections.
HIV, the Cause of AIDS
In 1983, Dr. Luc Montagne and colleagues at the Institute Pasteur in France reported that they had isolated a new virus called lymphadenopathy-associated virus, or LAV, which was the cause of AIDS. A short time later, in 1984, Dr. Robert Gallo and colleagues at the National Cancer Institute (NCI) reported that they had isolated a virus called HTLV-III, which was believed to cause AIDS. Both were right, as the virus from French and American groups were nearly identical. Sometime later the virus was renamed human immunodeficiency virus, or HIV. It belongs to the family, or group, of retroviruses called lentiviruses, which are found in a wide range of nonhuman primates.
A retrovirus's genes are made up of RNA, and so they need to live in a host and “borrow” the host's DNA in order to multiply.
A retrovirus is a type of virus with RNA as its genetic material. These viruses can convert their RNA to DNA and insert themselves into their host genome. Once inserted, they can stay latent for long periods of time and reactivate by transcribing copies of themselves at any time.
The Koch Test
Nearly all scientists recognize that HIV is the cause of AIDS. However, a small and vocal minority of researchers have argued that AIDS is not consistent with a single disease caused by a single infectious agent.
German bacteriologist Robert Koch (1843-1910) developed a series of rules, or postulates, for deciding whether a microbe is responsible for a specific disease. These four postulates stipulate that an infectious agent must be found in all cases of the disease; the agent must be isolated from the host's body; the agent must cause disease when reintroduced into a healthy host; and the same agent must once again be isolated from the newly diseased host. As you'll see in the following examples, all four postulates have been fulfilled for HIV, convincingly identifying HIV as the cause of AIDS.
It takes several weeks for the body to respond to HIV infection and produce antibodies to fight the infection. Therefore, an antibody test to diagnose HIV will show a false negative if the infection is brand new.
Infection and Disease
Infection begins when an HIV particle encounters a specialized immune cell, a T cell. The virus particle attaches itself to the cell's outer surface membrane and enters the cell. Once in the cell, the virus releases its RNA, and a chemical converts it into HIV DNA. The new HIV DNA moves into the cell's nucleus and strong-arms the host cell to help make copies of the virus, releasing new infectious virus particles.
The rate of disease progression depends on several variables, including the health of the host and the particular strain of the virus. However, the median time from initial infection to the development of AIDS among untreated patients is typically 8 to 10 years.
HIV damages the body's sources of T cells and centers of immune activity, killing cells in the bone marrow and thymus that are needed for the development of mature immune cells. The virus also progressively destroys the lymph nodes, the centers of immune activity in the body. The viral load, or amount of virus in circulation, is a barometer of disease progression.
HIV infection is not like catching a cold or the flu, because it isn't spread by coughs or sneezes. You get HIV by coming in contact with infected blood, semen, or vaginal fluids from another person.
The relative risk of developing opportunistic infections is closely tied to T cell counts. As T cell counts go down, many different organisms can cause infections. Common AIDS-related opportunistic infections are caused by fungi like Candida (topical and systemic yeast infections), bacteria like tuberculosis, and viruses like hepatitis and herpes.
Early symptoms of HIV infection include …
A common physical manifestation of late-stage AIDS is the “Wasting Syndrome,” when body fat and body mass are significantly decreased, making a person look like they are suffering from severe malnutrition.
Excerpted from The Complete Idiot's Guide to Dangerous Diseases and Epidemics © 2002 by David Perlin, Ph.D., and Ann Cohen. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.