Also, this model does not work well for every disease. It works well where people recover from a disease and become immune, such as measles. Not all infectious diseases follow this pattern. For instance, people infected with HIV never recover, and there is no vaccine, so there is no group of recovered people, only susceptible and infected people. There is also the case where people recover from a disease but do not become immune or lose their immunity over time. For instance, when people get the flu, they are generally only immune for a short time before they become susceptible again.