INFECTIOUS DISEASE Infectious diseases are caused by microscopic organisms commonly called germs. Physicians refer to these disease-causing organisms as pathogens. Pathogens that infect humans include a wide variety of bacteria, viruses, fungi, protozoans, and parasitic worms. In addition, it has been theorized that some proteins called prions may cause infectious diseases. A Pathogens Chicken Pox Virus The spherical varicella-zoster virus (VZV) infects most children worldwide by the age of 10. Transmitted in airborne droplets exhaled from an infected person, the virus causes a low fever and a rash of fluid-filled blisters that begin as red spots covering most of the body and the inside of the mouth. The disease is dangerous to newborns, to people first infected in adulthood, and to those in whom the virus remains dormant in nerve cells, erupting as the more painful and sometimes chronic zoster (shingles) later in life. VZV is a member of the Herpes virus family, which also includes the causative agents of infectious mononucleosis, roseola, and oral and genital herpes.Martin Rotker/Phototake NYC Rabies Virus The rabies virus is usually transmitted to humans by a bite from an infected dog, but the bite of any animal (wild or domestic) is suspect in an area where rabies is present. In North America, skunks are the principal carriers of the disease, although the raccoon and bat populations are also affected. Symptoms of the disease appear after an incubation period of ten days to one year and include fever, breathing difficulties, muscle spasms, and in later stages, an irrational fear of water. Death almost invariably occurs within three days to three weeks of the onset of symptoms. For this reason, the emphasis of treatment is on prevention. In the United States, veterinarians recommend regular vaccination of domestic dogs.Tektoff-RM/CNRI/Science Source/Photo Researchers, Inc. Hepatitis B Virus The hepatitis B virus (HBV) causes inflammation of the liver. The virus is recognizable under magnification by the round, infectious “Dane particles” accompanied by tube-shaped, empty viral envelopes. Symptoms of hepatitis B infection include jaundice and a flulike illness, while chronic infection can lead to serious problems such as cirrhosis and cancer of the liver.Institute Pasteur/CNRI/Phototake NYC Viruses are tens or hundreds of times smaller than bacteria. They are not cellular, but consist of a core of genetic material surrounded by a protective coat of protein. Viruses are able to survive and reproduce only in the living cells of a host. Once a virus invades a living cell, it directs the cell to make new virus particles. These new viruses are released into the surrounding tissues, and seek out new cells to infect. The roll call of human diseases caused by viruses includes mumps, measles, influenza, rabies, hepatitis, poliomyelitis, smallpox, AIDS, and certain types of cancer.
Protozoans are single-celled, animal-like organisms that live in moist environments. Perhaps the most infamous pathogenic protozoans are species of the genus Plasmodium, which cause malaria, an infectious disease responsible for over 2 million deaths worldwide each year. Members of the genus Trypanosoma produce trypanosomiasis, also known as African sleeping sickness, and Chagas' disease. Other protozoans cause giardiasis, leishmaniasis, and toxoplasmosis. Life Cycle of Human Blood Flukes Flukes of the genus Schistosoma parasitize two hosts. The young hatch from their eggs in rivers and lakes and enter a specific kind of aquatic snail, where they develop into tadpolelike larva called cercariae. When the cercariae leave the snail, they burrow through the skin of a human host swimming or wading in infested water. Adult flukes mature in the host’s bloodstream and settle in the veins of the gut. Their eggs, deposited in the lining of the human intestine and bladder, pass back into water via the sewage system, and the cycle begins again. More than 200 million people worldwide suffer from schistosomiasis, a disease characterized by the abscesses and bleeding caused by the flukes’ infestation.© Microsoft Corporation. All Rights Reserved. Roundworms, or nematodes, are small, tubelike worms that are pointed at both ends. Species that infect human intestines include pinworms, hookworms, threadworms, and members of the genus Ascaris. Trichinella spiralis can invade human muscle tissue, often from eating infected pork that has been improperly prepared, causing a disease called trichinosis. Prions are extremely tiny protein particles found in the brain, nerve, and muscle cells. A controversial theory states that prions cause disease by changing normal proteins into an abnormal shape. These mutated proteins in turn force other proteins to change shape, leading to destruction of tissue, primarily in the brain. Some researchers have hypothesized that prions cause transmissible spongiform encephalopathies, a group of rare infectious diseases that includes Creutzfeldt-Jakob disease in humans, scrapie in sheep, and bovine spongiform encephalopathy (commonly known as mad cow disease) in cattle. Some evidence suggests that prion-related disease can be transmitted through food infected with mutated proteins. B Spread of Infectious Disease South African Tsetse Fly The South African tsetse fly, common to parts of central Africa, is responsible for transmitting the parasitic protozoan that causes sleeping sickness, which can be fatal to humans and domestic cattle. The fly uses its mouth parts to bite and draw blood from its host. If it draws blood from an individual infected by sleeping sickness, the fly can then transmit the infection to other hosts in subsequent bitings.David Shale/Oxford Scientific Films Other pathogens involve an intermediary carrier, such as an insect. The malarial parasite, for example, spends part of its life cycle in mosquitoes, then enters a person's bloodstream when the mosquito bites the person. Many pathogens are spread through contaminated food and water. Cholera bacteria, for example, are spread through food and water contaminated with the excrement of infected people. C New Infectious Diseases Life Cycle of Human Immunodeficiency Virus The human immunodeficiency virus (HIV), the cause of acquired immunodeficiency syndrome (AIDS), is genetically programmed to do one thing: highjack the reproductive machinery of a human cell, then trick it into churning out as many copies of the virus as it can before the cell dies. The current best hope for the treatment of AIDS requires that patients take a number of different drugs, each of which interferes with certain steps of the HIV infection process.© Microsoft Corporation. All Rights Reserved. sidebar Among the diseases new to science are AIDS, Ebola hemorrhagic fever, Legionnaires’ disease, and Lyme disease. AIDS has been the most deadly of all the new diseases, but even it has not taken as high a toll as malaria, tuberculosis, and other diseases that have been around for centuries. Some newly identified disease-causing agents for diseases that have been recognized for a long time include Human T-lymphotropic virus I (HTLV-1), which can cause some cases of non-Hodgkin’s lymphoma, a type of cancer originating in the lymphatic system; and HTLV-2, which is associated with hairy-cell leukemia, a rare type of cancer of the blood. In most cases, the reasons for the emergence of a new disease are unknown. One exception is Legionnaires’ disease. It is caused by a bacterium that was not identified until after an outbreak in 1976 at an American Legion convention in Philadelphia, Pennsylvania. Once identified, however, scientists were able to retrospectively identify earlier epidemics of the disease, and realized that each year the bacterium is responsible for thousands of cases of pneumonia.
In addition to new diseases, well-known pathogens may change, or mutate, creating new, virulent strains. Influenza viruses are among those that mutate frequently, which explains why flu shots—vaccines that use modified or killed versions of the influenza agent to stimulate a protective immune response in the body—are given annually, and why epidemics of influenza periodically occur. The strains of flu virus that were most prevalent one year differ from those that bedevil humans the next year. Vaccines that protected against last year's flu virus may need to be altered to be effective against today's most common strains. A similar problem occurs when mutations in infectious agents result in resistance to medicines that had been effective treatments. The bacteria that cause bronchitis, meningitis, tuberculosis, and pneumonia are among many that have developed strains that are resistant to at least some antibiotics. As a result, doctors have fewer options for treating the diseases and preventing their spread.
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Viruses are particles composed of genetic material (DNA or RNA, but not both) surrounded by a protective protein coat. Outside a host cell, they are inert; inside, they enter a dynamic phase in which they replicate, pirating the host cell’s enzymes, nucleic and amino acids, and machinery to accomplish what they are not equipped to do alone. Viral replication is often carried out at the expense of the host: Diseases such as herpes, rabies, influenza, some cancers, poliomyelitis, and yellow fever are of viral origin. Of the estimated 1000 to 1500 types of viruses, approximately 250 cause disease in humans (over 100 of which cause the common cold), and 100 infect other animals. 
Helminth Worm Helminth worms attack the digestive tract and other internal organs of the vertebrate body and cause a wide variety of parasitic diseases. These worms include such diverse forms as the roundworms (nematodes), flukes (trematodes), tapeworms (cestodes), thorny-headed worms (acanthocephalans), and tongue worms (linguatulids). Many species are only able to complete their life cycle by spending time in a variety of animal hosts. 
Ebola Virus Infection The Ebola virus is highly infectious and can spread through the use of unsterilized needles or through contact with an infected individual or the corpse of someone who has died from the disease. About one week after infection, the virus begins attacking blood and liver cells (1). As the disease swiftly progresses, the virus may destroy vital organs such as the liver and kidneys (2), leading to massive internal bleeding (3). Shock and respiratory arrest soon follow, then death.© Microsoft Corporation. All Rights Reserved.  |
