Communicable Diseases

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Communicable diseases

A communicable disease is any disease that can be transmitted from one organism to another. Agents that cause communicable diseases, called pathogens, are easily spread by direct or indirect contact. These pathogens include viruses, bacteria, fungi , and parasites . Some pathogens make toxins that harm the body's organs. Others actually destroy cells. Some can impair the body's natural immune system, and opportunistic organisms set up secondary infections that cause serious illness or death. Once the pathogens have multiplied inside the body, signs of illness may or may not appear. The human body is adept at destroying most pathogens, but the pathogens may still multiply and spread.

The pathogens responsible for some communicable diseases have been known since the mid-1800s, although they have existed for a much longer period of time. European explorers brought highly contagious diseases such as smallpox, measles, typhus and scarlet fever to the New World, to which Native Americans had never been exposed. These new diseases killed 50–90% of the native population. Native populations in many areas of the Caribbean were totally eliminated.

In some areas of the world, contaminated soil or water incubate the pathogens of communicable diseases, and contact with those agents will cause diseases to spread. In the 1800s, when Robert Koch discovered the anthrax bacillus (Bacillus anthracis ), cholera bacillus (Vibrio cholerae ), and tubercle bacillus (Mycobacterium tuberculosis ), his work ushered in a new era of public sanitation by showing how water-borne epidemics, such as cholera and typhoid, could be controlled by water filtration .

Malaria , another communicable disease, was responsible for the decline of many ancient civilizations; for centuries, it devitalized vast populations. With the discoveries in the late 1800s of protozoan malarial parasites in human blood, and the discovery of its carrier, the Anopheles mosquito, malaria could be combatted by systematic destruction of the mosquitos and their breeding grounds, by the use of barriers between mosquitos and humans such as window screens and mosquito netting, and by drug therapy to kill the parasites in the human host.

Discoveries of the causes of epidemics and transmissible diseases led to the expansion of the fields of sanitation and public health. Draining of marshes, control of the water supply, widespread vaccinations, and quarantine measures improved human health. But, despite the development of advanced detection techniques and control measures to fight pathogens and their spread, communicable diseases still take their toll on human populations. For example, until the 1980s tuberculosis had been declining in the United States due in large part to the availability of effective antibiotic therapy. However, since 1985, the number of tuberculosis cases has risen steadily due to such factors as the emergence of drug-resistant strains of the tubercle bacillus, the increasing incidence of HIV infection which lowers human resistance to many diseases, poverty, and immigration.

Epidemiologists track communicable diseases throughout the world, and their work has helped to eradicate smallpox, one of the world's most deadly communicable diseases. A successful global vaccination campaign wiped out smallpox in the 1980s, and today, the virus exists only in tightly-controlled laboratories in Moscow and in Atlanta at the Centers for Disease Control and Prevention . Scientists are currently debating whether to destroy the viruses or preserve them for study.

Communicable diseases continue to be a major public health problem in developing countries. In the small West African nation of Guinea-Bissau, a cholera epidemic hit in 1990. Epidemiologists traced its outbreak to contaminated shellfish and managed to control the epidemic, but not before it claimed hundreds of lives in that country. Some victims had eaten the contaminated shellfish, others were infected by washing the bodies of cholera victims and then preparing funeral feasts without properly washing their hands. Proper disposal of victims' bodies, coupled with a campaign to encourage proper hygiene, helped stop the epidemic from spreading.

Communicable diseases can be prevented either by eliminating the pathogenic organism from the environment (as by killing pathogens or parasites existing in a water supply) or by placing a barrier in the path of its transmission from one organism to another (as by vaccination or by isolating individuals already infected). But identifying and isolating the causal agent and developing weapons to fight it is time consuming, and, as with the AIDS virus, thousands of people continue to become infected and many die because educational warnings about ways to avoid infection frequently go unheeded.

AIDS is caused by the human immunodeficiency virus (HIV). Spread by contact with bodily fluids of an HIV-infected person, the virus weakens and eventually destroys the body's immune system. Researchers think the virus originated in monkeys and was first transmitted to humans about 40 years ago. African chimpanzees can be infected with HIV, but don't develop AIDS. This suggests questions that are so far unanswered: Was there a genetic change in the virus, or was there simply more contact between monkeys and people as human populations encroached on their habitat ?

Some scientists think the AIDS pandemic is just the tip of the iceberg. They worry that new diseases, deadlier than AIDS, will emerge. Virologists point out that viruses, like human populations, constantly change. Rapidly increasing human populations provide fertile breeding grounds for microbes , including viruses and bacteria. Pathogens can literally travel the globe in a matter of hours.

For example, the completion in 1990 of a major road through the Amazon rain forest in Brazil led to outbreaks of malaria in the region. In 1985, used tires imported to Texas from eastern Asia transported larvae of the Asian tiger mosquito, a dangerous carrier of serious tropical communicable diseases. Deforestation and agricultural changes can unleash epidemics of communicable diseases. Outbreaks of Rift Valley fever followed the construction of the Aswan High Dam , most likely because breeding grounds were created for mosquitoes which spread the disease. In Brazil, the introduction of cacao farming coincided with epidemics of Oropouche fever, a disease linked to a biting insect that thrives on discarded cacao hulls.

Continued rapid transportation of humans around the world is likely to accelerate the movement of communicable diseases. Poverty, lack of adequate sanitation and nutrition, and the crowding of people into megacities in the developing countries of the world only exacerbate the situation. The need for study and control of these disease is likely to grow in the future.

[Linda Rehkopf ]