The term contagious distribution is apparently first used by Neyman (Ann. Math. Stat. 10:35–57,1939) for a discrete distribution that exhibits clustering or contagious effect. The classical Neyman Type A distribution is one well-known example. However, contagious distributions are used nowadays to describe a plethora of distributions, many of which possess complicated probability distribution expressed in terms of special functions (see, for instance, Johnson et al., Univariate Discrete Distributions, 3rd edn. Wiley, Hoboken, 2005).