This is termed the SIR model. Individuals are born into the susceptible class. Susceptible individuals have never come into contact with the disease and are able to catch the disease, after which they move into the infectious class. Finally, individuals in the recovered class are assumed to be immune for life. Therefore, although initially this model shows large epidemics occurring at regular intervals,eventually the level of the disease reaches a constant value.
Hyaline membranes are present, preventing alveolar oxygen exchange resulting in respiratory failure.
Many interesting and useful results have been proved for the simple SIR model, but before we can explore this rich subject area, we need a further bit of Epidemiological notation. One fundamental parameter governs the spread of diseases, and is also related to the long term behaviour and the level of vaccination necessary for eradication. This parameter is called the basic reproductive ratio, R 0 . As such R 0 tells us about the initial rate of increase of the disease over a generation. When R 0 is greater than 1, the disease can enter a totally susceptible population and the number of cases will increase, whereas when R 0 is less than 1, the disease will always fail to spread.