3.1 Birds

First, we considered the bird population in New York City during the 2000 West Nile outbreak. All the birds in that population can be classified into one of two groups: those that were infected with West Nile virus, and those that were not. (The third group of birds, the dead ones, don’t influence the disease dynamics.) In epidemiological terminology, the birds that have West Nile virus are called infectious, and those that don’t have it are called susceptible. So, our model needed to keep track of the number and fate of infectious and susceptible birds.