Susceptibility

Genetic Certain diseases can only affect animals and when they are transmitted to man, they are not able to establish themselves. An example is Plasmodium berghei, the rodent malaria parasite, which cannot produce disease in man although closely related to the human malaria parasite. However, some newly emergent diseases have succeeded in crossing this genetic barrier, such as HIV and severe acute respiratory syndrome (SARS).

Genetic disposition also determines the host's response to infecting organisms. Mycobacteria are common in the environment, but only certain people develop tuberculosis or leprosy. The type of disease (e.g. tuberculoid or lepromatous leprosy), is also determined by the genetic make-up.

Age During the course of life, different diseases affect particular age groups. The childhood diseases of measles, chickenpox and diphtheria are found at one end of the lifespan, with the degenerative diseases and neoplasms predominating at the other.

Sex The same advantages that parasites derive from having two sexes, producing many individuals of different vigour, also benefit humans, and it is thought that evading parasites might be one of the main reasons why mammals evolved with two sexes. However, one or other sex might more commonly succumb to illness, such as poliomyelitis and goitre, which are more commonly found in females than males. Occupation can determine which sex is more likely to be involved, such as in East Africa where males, who hunt and collect honey in tsetse fly infested forest, are more likely to contract sleeping sickness. Social habits may also be a determinant, such as the custom of the Fore people in Papua New Guinea, where the women eat the brains of the recently dead, making kuru predominantly a disease of women.

Pregnancy When a woman is pregnant, her physiological mechanisms are altered and she becomes more susceptible to infections. Chickenpox is a severe disease in pregnancy and malaria attacks the pregnant woman as though she had little acquired immunity. The pregnant woman contracting Lassa fever is more likely to die from the illness.

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