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This outbreak of measles begins with a single index case that infects a number of other individuals. (The incubation period for measles averages 10 days with a range of 7-18 days.) One or more of the people infected in the initial wave infects a group of people who become the second wave of infection. So here transmission is person-to-person, rather than from a common source.
This is a propagated (progressive source) epidemic in which one or more of the first wave of cases serves as a source of infection for subsequent cases and those subsequent cases, in turn, serve as sources for later cases. The shape of the curve usually contains a series of successively larger peaks, which are one incubation apart in which an increasingly large number of cases caused by person-to-person contact, until the pool of susceptibles is exhausted or control measures are implemented. In reality, most of these epidemics do not produce the classic pattern.
[Source: CDC website on interpreting an epidemic curve]
