Diseases go through five stages: incubation period, prodromal period, period of illness, period of decline, and period of convalescence.
Infections progress through a common pattern. The severity and duration of the steps in the pattern may vary among pathogens and hosts to some degree, but the overall pattern is similar. Differences in severity and duration of the stages in disease progression are often of diagnostic value. After initial invasion of the host, there is an incubation period followed by the prodromal period, the period of illness, the period of decline, and finally the period of convalescence before the infection is completely cleared.
The incubation period is the initial stage of the disease process before symptoms become apparent and the pathogen is actively replicating. There are no signs or symptoms during the incubation period. Incubation times vary widely and are influenced by host health, the pathogen transmission route, and the number of pathogens invading. Lyme disease, caused by the protist Borrelia burgdorferi, is transmitted to humans through the bite of infected ticks. Typical symptoms include fever, headache, and extreme tiredness. In Lyme disease, the incubation begins after infection of the host by a tick bite and before any symptoms of disease are shown. In rapidly progressing acute infections, the incubation may last only hours, while in slow-to-develop chronic diseases, such as HIV, the incubation period may last years. The prodromal phase is the stage of the disease process when symptoms first become apparent. These symptoms are typically unspecific to the pathogen and vague; they may include fever, fatigue, and headaches. As the pathogen population continues to grow, the period of illness sets in and is marked by more severe symptoms and the initiation of an immune response. As the pathogen replicates, the aggregate effects of pathogen toxin release, host cells lysis, and immune response to the infection begin to damage host tissues and cause localized inflammation. The specific symptoms and their localizations in the period of illness provide diagnostic information. In Lyme disease the prodromal phase corresponds to both the early localized and acute disseminated stages of the disease, which are when the symptoms of a skin rash (erythema migrans) and meningeal irritation (headache) become apparent. The host has a generalized feeling of being unwell. This includes symptoms of fever, fatigue, sore throat, swollen lymph nodes, chills, and muscle aches. The decline phase is the stage of disease when symptoms begin to abate and the pathogen population begins to decline. Damaged tissues begin to repair, and pathogen numbers decrease. Some hosts infected with Lyme disease enter the decline phase without treatment, but most require antibiotic treatment during the early localized or the acute disseminated stages of the disease to enter the decline phase. For those who do not receive treatment or enter into decline phase on their own, stage 3, or late disseminated Lyme disease, occurs. This can be months to years after the initial tick bite. During this stage, the infected host develops arthritis, heart rhythm disturbances, memory loss, encephalitis, numbness in the extremities, and severe headaches. The period of decline transitions into the convalescence period, which is the stage of the disease process when symptoms disappear. It is considered a recovery period, when host strength is regained. Host tissues are repaired to their preinfection health. The pathogen is eliminated (or its population is reduced below clinical significance), and the infection is resolved. Some hosts infected with Lyme disease are able to reach the convalescence period without treatment, but most require the disease to be diagnosed and treated with antibiotics during the first two stages of the disease: early localized or acute disseminated. In those hosts not treated and unable to resolve the infection, chronic disease ensues with stage 3 or late disseminated disease.