By Larry Romanoff, December 14, 2020
Let's examine the normal pattern for an outbreak of a typical infectious disease. According to the US CDC: (1)
"A common-source outbreak is one in which a group of persons are all exposed to an infectious agent or a toxin from the same source. If the number of cases during an epidemic were plotted over time, the resulting graph . . . would typically have a steep upslope and a more gradual downslope (a so-called “log-normal distribution”). A propagated outbreak results from transmission from one person to another [usually] by direct person-to-person contact . . ." Propagated outbreaks typically exhibit several peaks one or two weeks apart, the epidemic normally dying out after several of these generations.
"Some epidemics have features of both common-source epidemics and propagated epidemics. The pattern of a common-source outbreak followed by secondary person-to-person spread is not uncommon." The CDC states these also can produce several generations or peaks during the following few weeks. But in all of these instances of natural infectious agent outbreaks, the spread and timing follow essentially the same typical pattern, perhaps elongated but still with close timing of the peaks. Here are three graphs from the CDC to illustrate. You can see clearly that we have a rise (rapid if single-source, slow if propagated or mixed), then a peak, a gradual
tapering-off, and a cessation.
The Dreaded "Second Wave"
