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When faced with a perplexing patient a clinician may find the fever curve to be easily obtainable and generally reliable as a guide to subsequent investigations. Similarly the age distribution of ill persons in a population may provide the epidemiologist with reliable data from which to speculate on the general characteristics of an unknown disease. The following article illustrates the use of the age distribution to categorize a new epidemic and to guide further investigations.From January 6 to June 20, 1995, 315 persons from Kikwit and the surrounding Bandundu region of Zaire developed Ebola hemorrhagic fever; 244 (77%) died.1 Clinical features included fever, headache, sore throat, diarrhea and myalgia, followed in the later stages by vomiting, worsening diarrhea and shock. Approximately one-third of patients had clinical signs of hemorrhage at some point during their disease.2One of the unique epidemiologic features of this outbreak was the relative sparing of the pediatric population. Of the 315 patients only 27 (9%) were 17 years of age or younger. Such an age distribution is unexpected, particularly in the developing world, where a large proportion of the population is comprised of children. In Zaire 50% of the population is <16 years of age, in contrast to the United States, where this proportion is 23%.3The predominance of adults among patients with Ebola hemorrhagic fever in Kikwit provided a clue from which speculation could be made about the epidemiology of the disease, based on generalizations drawn from experience with the age distributions of other infectious diseases. Although some descriptive data existed from a handful of previous outbreaks, including the 1976 outbreak in Yambuku, the international team that responded to the request for assistance from the government of Zaire lacked important basic knowledge about Ebola hemorrhagic fever. On the basis of the age distribution in Kikwit several hypotheses could be generated: the population lacked preexisting immunity to the agent and the agent was not highly transmissible; children either were not exposed to the agent or were less susceptible to severe disease; or less likely, this was a sexually transmitted agent or one with an extremely long incubation period.Children often lack specific immunity to common pathogens; therefore they bear the greatest burden of illness from infectious diseases. Diseases of childhood, such as measles, chickenpox, erythema infectiosum and others, tend to spare adults because almost all members of the population are infected as children, acquired immunity to these infections is protective and this protection lasts for decades. Even infectious diseases such as malaria and respiratory syncytial virus bronchiolitis, for which immunity is believed to be only partially protective, cause the greatest morbidity and mortality in young children. One possible explanation for a predominantly adult distribution of an infectious disease is that the population is immunologically naive to the agent. This would be the case with a recently introduced pathogen, or one for which survival with immunity is rare.However, an immunologically naive population cannot fully account for a predominance of adult patients, because highly transmissible agents introduced into such populations do not result in the age distribution seen in Kikwit. Measles virus, the prototype of a highly transmissible agent, was introduced into the population of the Faroe Islands in 1846 for the first time in 65 years. Of the 7782 inhabitants more than 6000 became infected, without selection of any particular age group.