A situation like this is South America at the time of the Spanish Conquest, when smallpox was introduced into the native population, or the new varieties of flu that crop up every several years. Another situation these days is the introduction of a new virus into the computer community. We will keep track of the number of individuals in each group. Although these numbers should be integers, it is convenient to use real numbers. Susceptible individuals are those that have never had the disease.
Sick individuals have the disease at present.
Some sick individuals will die. Cured individuals have had the disease, and are now immune. The numbers in each group will change from day to day, and the total population will decrease as people die. If the time span is short enough, births of new individuals can be disregarded. The number of people in the susceptible group will decrease in each time period as individuals in that group get sick.