advance—the microscope—perfected in the 1600s allowed for their discovery, and so they are called microparasites. The ultimate microparasite is a virus—a small piece of nucleic acid (RNA or DNA) enclosed within a protein coat. A virus has no cell membrane, no cytoplasm, and no organelles; and because it has no metabolic machinery of its own, it requires a living cell to make more virus. Viruses are <0.1 µm in size; they cannot be seen even with the light microscope, but only with the electron microscope, which can magnify objects >10,000 times. Viruses, such as the agents of SARS, AIDS, Zika, Ebola, yellow fever, and the flu, are neither cells nor organisms.
Microparasites reproduce within their hosts and are sometimes referred to as infectious microbes, or, more commonly, “germs.” Larger parasites, ones that can be seen without the use of a microscope, are referred to as macroparasites; they are composed of many cells. Those that most often cause diseases of humans or domestic animals are roundworms, such as the hookworm; flatworms, such as the blood fluke; blood-sucking insects, such as mosquitoes, flies, and lice; or arachnids, such as ticks. Macroparasites do not multiply within an infected individual (except in the case of larval stages in the intermediate hosts) but instead produce infective stages that usually pass out of the body of one host before transmission to another host.
“What’s in a name? That which we call a rose by any other name would smell as sweet.” When William Shakespeare penned these lines in Romeo and Juliet, he gave value to substance over name-calling. But being able to tell one microbe from another is more than having a proper name for a germ—it can have practical value. Imagine you have just returned from a trip and now suffer with a fever, headache, and joint pains, and worst of all you have a severe case of diarrhea. What a mess you are! When you see your physician, she tells you that the cause of your distress could be due to an infection with Salmonella or Giardia or Entamoeba or the influenza or SARS virus. Prescribing an antibiotic for diseases caused by a virus would do you no good, but for “food poisoning” caused by Salmonella, a bacterium, a course of antibiotic therapy might restore you to health. On the other hand, if your clinical symptoms were due to the presence of protozoan parasites such as Giardia or Entamoeba, they would not respond to antibiotics either, and other drugs would have to be prescribed to cure you. Determining the kind of parasite (or parasites) you harbor, therefore, will do more than provide the name of the offender; it will allow for the selective treatment of your illness.
Plagues and Parasites
In antiquity, all disease outbreaks, irrespective of their cause, were called plagues; the word “plague” comes from the Latin plaga, meaning “to strike a blow that wounds.” When a parasite invades a host, it establishes an infection and wounds the body (Fig. 1.2). Individuals who are infected and can spread the disease to others (such as SARS patient 4) are said to be contagious or infectious. Initially, Legionnaires’ disease and TSS were thought to be contagious. Despite the obvious clinical signs of coughing, nausea, vomiting, and diarrhea, however, a person-to-person-transmissible agent was not found. In short, the victims of TSS and Legionnaires’ disease were not infectious, in contrast to what we know in cases of influenza, SARS, and the common cold with a similar array of symptoms. Influenza and SARS are different kinds of diseases of the upper respiratory system: the flu is contagious 24 h before symptoms appear, has a short (2-to-4-day) incubation period, and requires hospitalization infrequently; whereas SARS has a longer (3-to-10-day) incubation period, the patient is infectious only after symptoms appear, and the infection requires that the victim be hospitalized.
Infectiousness, however, may persist even after disease symptoms have disappeared; such infectious but asymptomatic individuals are called carriers. The most famous of these carriers was the woman called “Typhoid Mary,” an Irish immigrant to the United States whose real name was Mary Mallon. In 1883 she began working as a cook for a wealthy New York banker, Charles Henry Warren, and his family. The Warren family rented their large house in Oyster Bay, Long Island, from a George Thompson. That summer, six of eleven people in the house came down with typhoid fever (caused by the “germ” Salmonella typhi), including Mrs. Warren, two daughters, two maids, and a gardener. Mr. Thompson, fearing he would be unable to rent his “diseased house” to others, hired George Soper, a sanitary engineer, to find the source of the epidemic. Soper’s investigation soon led him to Mary Mallon, who had been hired as a cook just 3 weeks before the outbreak of typhoid in the Warren household. Mary had remained with the Warrens for only a month and had already taken another position when Soper found her. On June 15, 1907, Soper published his findings in the Journal of the American Medical Association: Mary was a healthy carrier of typhoid germs. Although she was unaffected by the disease (which causes headache, loss of energy, diarrhea, high fever, and, in a tenth of cases, death), she still could spread it. When Soper confronted Mary and told her she was spreading death and disease through her cooking, she responded by seizing a carving fork, rushing at him, and driving Soper off. Soper, however, was undaunted and convinced the New York City Health Department that Mary was a threat to the public’s health. She was forcibly carried off to an isolation cottage at Riverside Hospital on Rikers Island in the Bronx. There, her feces were examined and found to contain the typhoid bacteria. Mary remained at the hospital, without her consent, for 3 years and then was allowed to go free as long as she remained in contact with the Health Department and did not engage in food preparation. She disappeared from the Health Department’s view for a time but then took employment as a cook at the Sloane Maternity Hospital under an assumed name, Mrs. Brown.
During this time she spread typhoid to 25 doctors, nurses, and staff, 2 of whom died. She was sent again to Rikers Island, where she lived the rest of her life, 23 years, alone in a one-room cottage. During her career as a cook, “Typhoid Mary” probably infected many more than the 50 documented cases, and she surely caused more than 3 deaths. Mary Mallon was not the only human carrier of typhoid. In 1938 when she died, the New York City Health Department noted that there were 237 others living under their observation. She was the only one kept isolated for years, however, and one historian has ascribed this to prejudice toward the Irish and a non-compliant woman who could not accept that unseen and unfelt “bugs” could infect others. Mary Mallon told a newspaper: “I have never had typhoid in my life and have always been healthy. Why should I be banished like a leper and compelled to live in solitary confinement … ?”
Predicting Plagues
Recognizing the elements required for a parasite to spread in a population allows for better forecasting of the course a disease may take. Three factors are required for a parasite to spread from host to host: there must be infectious individuals, there must be susceptible individuals, and there must be a means for transmission between the two. Transmission may be by indirect contact involving vectors such as mosquitoes (in malaria and yellow fever) or flies (in sleeping sickness and river blindness) or ticks (in Lyme disease), or it may be by direct contact as it is with measles, influenza, SARS, and tuberculosis, where it is influenced by population density.
In the past, the sudden increase in the number of individuals in a population affected by a disease was called a plague. Today we frequently refer to such a disease outbreak as an epidemic, a word that comes from the Greek epi, meaning “among,” and demos, “the people.” Epidemiologists are disease forecasters who study the occurrence, spread, and control of a disease in a population, using statistical data and mathematical modeling to identify the causes and modes of disease transmission and to predict the likelihood of an epidemic, to identify the risk factors, and to help plan control programs such as quarantine and vaccination. When TSS broke out, epidemiologic studies linked the syndrome to the use of tampons, principally Rely tampons, and the recommendation was that the illness could be controlled in menstruating women by the removal of such tampons from the market. Acting on this advice, Procter & Gamble stopped marketing Rely tampons and the number of cases virtually
