Chapter 2-4 Which of the following is a condition which may occur during the incubation period? • Transmission of infection The incubation period is defined as the interval from receipt of infection to the time of onset of clinical illness. Accordingly, individuals may transmit infectious agents during the incubation period as they show no signs of disease that would enable the isolation of sick individuals by quarantine. Chicken pox is a highly communicable disease. It may be transmitted by direct contact with a person infected with the varicella-zoster virus (VZV). The typical incubation time is between 10 to 20 days. A boy started school 2 weeks after showing symptoms of chicken pox including mild fever, skin rash, and fluid-filled blisters. One month after the boy returned to school, none of his classmates had been infected by VZV. The main reason was: • Contact was after infectious period • Subclinical infections were not yet detected • Disease was endemic in the class The disease is spread by contact with an infected individual who can transmit the agent (VZV) to immunologically naive persons during the incubation period and for several days after onset of clinical illness. Since the boy started school 14 days after showing signs consistent with chicken pox, it is most likely that he was no longer infectious. The ability of a single person to remain free of clinical illness following exposure to an infectious agent is known as: • Hygiene • Vaccination • Herd immunity • Immunity • Latency Immunity is the capacity of a single individual to avoid disease susceptibility when exposed to an infectious agent. Herd immunity is a population characteristic. For certain diseases, individual immunity can be acquired by vaccination, but this is not true for all infectious diseases. Which of the following is characteristic of a single-exposure, common-vehicle outbreak? • Long latency period before many illnesses develop • There is an exponential increase in secondary cases following initial exposures • Cases include only those who have been exposed to sick persons • The epidemic curve has a normal distribution when plotted against the logarithm of time • Wide range in incubation times for sick individuals Single-exposure, common-vehicle outbreaks involve a sudden, rapid increase in cases of disease that are limited to persons who share a common exposure. Additionally, few secondary cases develop among persons exposed to primary cases. A histogram of the outbreak can plot the number of cases by time of disease onset. In single-exposure, common-vehicle outbreaks, a log transformation of the time of disease onset will often take on the characteristic shape of a normal distribution (i.e., a bell curve) with the median incubation time found at the peak of the curve. What is the diarrhea attack rate in persons who ate both ice cream and pizza? • 39/52 • 21/70 • 39/67 • 51/67