In order for a pathogen to infect a host, there must be a susceptible host for the disease to be transmitted. If a host's resistance is low (resistance is the ability to ward off disease), its susceptibility increases (its chances of becoming infected increase). Primary defense mechanisms of the body for resistance include intact skin (no cuts or abrasions), mucous membranes, a good cough reflex, normal gastric juices, and normal bacterial flora.
If a microorganism penetrates these defenses the development of a disease process begins. First there may be an incubation period. This is the time between the initial exposure and start of the infection to the first appearance of signs and the feeling of symptoms. Different microorganisms have different incubation periods. An examples is the Epstein-Barr virus, which causes infectious mono-nucleosis and has an incubation period of two to six weeks.
The varicella-zoster virus, which causes varicella (chickenpox), has an incubation period of two weeks. The human immunodeficiency virus, the virus that causes AIDS, has an incubation period of 7 to 11 years. During this phase, the disease can be spread from the infected individual to a non-infected individual.
The prodromal period follows the incubation period. This period presents with mild symptoms.
The period of illness is the acute phase of the disease. Here the individual presents with signs and symptoms of the disease. Signs are objective findings that an observer or physician can see. These are physical changes that can be measured. Examples of signs are fever, skin color or lesions, blood pressure, inflammation, and paralysis.
Symptoms are subjective and cannot be seen by an observer. Symptoms present as changes in bodily functions, such as pain, numbness, chills, general fatigue, or gastrointestinal discomfort. It is in this period of the disease where white blood cells may increase and the individual's immune system responds to combat the disease-causing pathogen. If the individual's defense mechanism of the immune system does not successfully overcome the disease or if the disease is not treated properly, the person can die.
During the period of decline, the individual's signs and symptoms subside and the person feels better. This period may take 24 hours to several days. During this time, the individual is prone to secondary infections.
The period of convalescence is the phase where recovery has occurred. The body regains strength and is returned to a state of normality. During this phase, infection can also be spread.
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