How do diseases spread?


22.4 How do diseases spread?

  • Context: Some unicellular organisms cause disease in ecological systems.
  • Major themes: All cells come from preexisting cells, cells maintain internal environments that differ from their external environments, cell structure defines cell function, and cells communicate with other cells.
  • Bottom line: Unicellular organisms that cause disease can spread rapidly in a susceptible population of animals, leading to an outbreak.
Biology Learning Objectives

  • Describe how severe acute respiratory syndrome (SARS) spreads.
  • Explain how disease outbreaks occur in ecological systems.

There are many species interactions in ecological systems. In parasite-host interactions, populations of animals or plants may be susceptible to newly-introduced disease-causing organisms. The pathogen may be a unicellular organism, such as Giardia or a bacterium. You already know how some parasites that cause disease (such as, Giardia and B. burgdorferi, which causes Lyme disease) affect individuals and populations. {Connections: Lyme disease is discussed in Section 22.2.} In this section, you will learn how a newly introduced pathogen spreads among the individuals in a population.

Figure 22.18 The SARS coronavirus, showing the single-stranded RNA (ssRNA) and four structural proteins: spike, which forms the corona; envelope; matrix; and nucleocapsid. Content Providers(s): CDC/Dr. Fred Murphy. This media comes from the Centers for Disease Control and Prevention’s Public Health Image Library (PHIL), with identification number #4814. Public domain.

Occasionally, new diseases emerge in a population. In late 2002, a farmer in Guangdong Province, in southern China, was treated for pneumonia with flu-like symptoms. The patient died, but no definitive diagnosis was made on the cause of death. By early 2003, many more cases were observed. The illness usually began with high fever and mild respiratory symptoms, but progressed to pneumonia in a few days. The disease was named severe acute respiratory syndrome (SARS). Because of the outbreak and severity of the disease, scientists quickly went to work to discover the causative agent of SARS, which turned out to be a coronavirus (Figure 22.18). Coronaviruses are enveloped by spherical spike proteins with surface projections that form a corona around the virus and a ssRNAcontaining (in the case of the SARS coronavirus) 13 genes that code for 14 proteins.

Figure 22.19 SARS outbreaks in Guangdong Province, China between November 2002 and February 2003. Each bar occurs at the onset of the outbreak for each city. The blue map shows China, with Guangdong Province in dark blue. Numbers in parentheses indicate cases in affected cities. Data from Zhong et al., 2003, Figure 1; original graph.

Y. Guan and his colleagues studied the epidemiology of the first outbreaks in Guangdong Province by gathering data from clinical records of patients admitted to hospitals with a diagnosis of atypical pneumonia, meaning that it was not caused by the bacteria Streptococcus pneumonia. In addition, the researchers used local health authority reports and doctors interviews to assess the spread of the disease (Figure 22.19).

The researchers defined a case as a patient running a high fever (> 38° C) with coughing or difficulty breathing who was either in close contact with a person suspected of having SARS or lived or traveled in an area known to have recent local transmission of SARS. Any patients that died from an unexplained acute respiratory illness who had come in close contact or traveled in an area known to have SARS cases were defined as a suspected case. Out of 305 cases during this early outbreak, 105 of the patients were healthcare workers. Patients suffering complications were sometimes brought from rural outlying cities to Guangzhou for better medical care. Many of these initial patients were known to be related to one another. One SARS patient, admitted on December 17, 2002, was a chef who worked at a restaurant in Shenzhen. This chef came into regular contact with live caged animals used as exotic game food. He was the second confirmed case of SARS.

The scientists also studied 55 patients admitted to a hospital in Guangzhou, Guangdong Province, in early 2003 with atypical pneumonia. These patients ran fevers for an average of 11.4 days (+ 6.8 days, standard deviation) and developed pneumonia within 4 days of being admitted. Forty-one of 55 were known to have had definitive contact with another SARS patient, and 27 were healthcare workers. Guan and his colleagues examined mucus and blood serum samples for the SARS coronavirus antibodies. For 22 of the patients, the scientists tested paired samples, the first either 3 to 5 or 7 to 10 days after disease onset, and the second 15 days after onset. The researchers also tested 60 healthy adults (Table 22.9). Four of the pneumonia patients who did not test positive for the SARS coronavirus antibodies were discovered to have influenza.


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