23–11 John Snow is widely regarded as the father of modern epidemiology. Most famously, he investigated an outbreak of cholera in London in 1854 that killed more than 600 victims before it was finished. Snow recorded where the victims lived, and plotted the data on a map, along with the locations of the water pumps that served as the source of water for the public (Figure Q23–2). He concluded that the disease was most likely spread in the water, although he could find nothing suspicious-looking in it. His conclusion ran counter to the then-current belief that cholera was from “miasmas” in bad air. Very few believed his theory during the next 50 years, with the “bad air” theory persisting until at least 1901. What do you suppose Snow saw in the data that led him to his conclusion? Why do you think most scientists remained skeptical for so long?
Figure Q23–2 a map of where the victims of the 1854 cholera outbreak lived, superimposed on a modern Google map of the area (Problem 23–11). The locations of the victims’ houses are indicated in a gradient of colors from blue (indicating few cases) to orange (indicating many cases). public water pumps are shown as red squares.
23–12 Influenza epidemics account for 250,000 to 500,000 deaths globally each year. These epidemics are markedly seasonal, occurring in temperate climates in the northern and southern hemispheres during their respec
Figure Q23–3 seasonal patterns of influenza epidemics (Problem 23–12). Cases of influenza at different times of the year are shown for the northern hemisphere (blue), the southern hemisphere (orange), and the tropics (red).
tive winters. By contrast, in the tropics, there is significant influenza activity year round, with a peak in the rainy season (Figure Q23–3). Can you suggest some possible explanations for the patterns of influenza epidemics in temperate zones and the tropics?
23–13 Several negative-strand viruses carry their genome as a set of discrete RNA segments. Examples include influenza virus (eight segments), Rift Valley fever virus (three segments), Hantavirus (three segments), and Lassa virus (two segments), to name a few. Why does segmentation of the genome provide a strong evolutionary advantage for these viruses?
23–14 Avian influenza viruses readily infect birds, but are transmitted to humans very rarely. Similarly, human influenza viruses spread readily to other humans, but have never been detected in birds. The key to this specificity lies in the viral capsid protein, hemagglutinin, which binds to sialic acid residues on cell-surface glycoproteins, triggering virus entry into the cell (Movie 23.8). Hemagglutinin on human viruses recognizes sialic acid in a 2-6 linkage with galactose, whereas avian hemagglutinin recognizes sialic acid in a 2-3 linkage with galactose. Humans make carbohydrate chains that have only the 2-6 linkage between sialic acid and galactose; birds make only the 2-3 linkage; but pigs make carbohydrate chains with both linkages. How does this situation make pigs ideal hosts for generating new strains of human influenza viruses?
23–15 The majority of antibiotics used in the clinic are made as natural products by bacteria. Why do you suppose bacteria make the very agents we use to kill them?
23–16 In the early days of penicillin research, it was discovered that bacteria in the air could destroy the penicillin, a big problem for large-scale production of the drug. How do you suppose this occurs?