Irwin W. Sherman - The Power of Plagues

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 … ?”

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 disappeared.

For an infection to persist in a population, each infected individual on average must transmit the infection to at least one other individual. The number of individuals each infected person infects at the beginning of an epidemic is given by the notation R 0; this is the basic reproductive ratio of the disease, or, more simply, the multiplier of the disease. The multiplier helps to predict how fast a disease will spread through the population.

The value for R 0can be visualized by considering the children’s playground game of touch tag. In this game one person is chosen to be “it,” and the objective of the game is for that player to touch another, who in turn also becomes ”it.” From then on each person touched helps to tag others. If no other player is tagged, the game is over, but if more than one other player becomes “it,” then the number of touch taggers multiplies. Thus, if the infected individual (it) successfully transmits the disease (touches another), then the number of diseased individuals (touch taggers) multiplies. In this example the value for R 0is the number of touch taggers that result from being in contact with “it.”

The longer a person is infectious and the greater the number of contacts that the infectious individual has with those who are uninfected, the greater the value of R 0and the faster the disease will spread. An increase in the population size or in the rate of transmission increases R 0, whereas an increase in parasite mortality or a decrease in transmission will reduce the spread of disease in a population. Thus, a change that increases the value of R 0tends to increase the proportion of hosts infected (prevalence) as well as the burden (incidence) of a disease. Usually, as the size of the host population increases, so do disease prevalence and incidence.