Why is a large population of hosts more important for the maintenance of a "crowd disease" than it is for other types of diseases?

"Crowd diseases" are acute in nature. Once a host is infected, the pathogen replicates rapidly, symptoms appear quickly, and the host usually either recovers relatively quickly and develops immunity to the disease, or dies. For the pathogen causing the disease, this means that if it spreads through a population, it can quickly "use up" its potential hosts, since most individuals are now either immune to a subsequent infection or dead. To survive, therefore, the pathogen requires a large population (a "crowd") of hosts, to insure there are always an adequate number of susceptible individuals in the population, in which the pathogen can reproduce. Other, chronic diseases do not necessarily require such large populations of hosts, because when a pathogen causing such a disease infects a host, the infection can last indefinitely, reducing the need for the pathogen to quickly find new hosts to infect.