A disease is holoendemic when essentially every individual in a population is infected.
Although the infection is ubiquitous, symptoms of disease do not appear equally across age groups. The young are more likely to express pathogenic responses, whilst the older hosts will carry the disease asymptomatically, or with reduced damage, due to adaptive immunity. Therefore, holoendemic diseases differ from hyperendemic diseases, of which symptoms are expressed equally by members across all age groups of a population. Note:"In the study area, like other holoendemic areas, youth is a risk factor for malaria. In comparison, adults in such areas have acquired permunition and can more readily resist infection and tolerate various symptoms associated with malaria."
Holoendemicity is frequently seen with malaria, specifically the strain caused by Plasmodium falciparum, in several regions of sub-Saharan Africa (one study found that 98.6% of the population had traces of the pathogen within a 4 month period Notes: "The incidence of malaria attacks was 40 times higher in children 0--4 years of age than in adults more than 40 years old. Our findings suggest that sterile immunity and clinical protection are never fully achieved in humans continuously exposed since birth to intense transmission."). While individuals of all ages risk exposure to malaria, those under the age of five are particularly susceptible to the disease. Children account for the majority of both local and global malaria casesWorld Malaria Report 2015. World Health Organization. December 2015. . Retrieved November 6, 2017. because they lack the adaptive immunity that comes with repeated exposure. Note: "From then on, it decreased with age (P < 0.001), first rapidly in children, then slowly in adults, as is classically observed in areas of malaria holoendemicity." Other examples of holoendemic diseases include ocular trachoma in certain areas in sub-Saharan Africa, where virtually all children in those populations have been infected, Notes: "Although the prevalence of active disease is similar for boys and girls, adult women tend to have more active disease than adult men, probably due to their more frequent interaction with children. In some areas trachoma is holoendemic—every child acquires active trachoma and every adult shows evidence of conjunctival scarring." and hepatitis B in areas of the Marquesas Islands.
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