Infectious diseases may affect not only individuals but also large groups of people or entire populations due to epidemic or highly endemic transmission. Throughout human history, a few microorganisms have been responsible for great epidemics and massive numbers of dead or crippled people as a result of infections spreading locally or throughout the world.28-31 Typhus has had a great impact. Typhus has been associated almost always with situations that involve overcrowding, famine, war, natural disasters, and poverty. The outcomes of several European wars were affected by the morbidity and mortality inflicted by typhus or other diseases on the military. Typhus epidemics were common during the world wars of the 20th century and in the concentration camps where the ecological conditions were ideal for such a disease to spread.30 Today, typhus and other rickettsioses are still public health problems in some countries, but overall the disease was brought under control after its life cycle was described and antibiotics, insecticides, and public health measures became available.30
Bubonic plague, caused by Yersiniapestis, is another disease that has shaped history, especially in Europe during the Middle Ages.31 Millions of people were affected by pandemics that spread throughout the continent. Tuberculosis, smallpox, and measles had a tremendous effect on the native populations of the Americas after Columbus's voyages to the New World. It has been estimated that 90% of the population in Mexico was killed by these pathogens, which were novel to the native residents.
Acquired immunodeficiency syndrome (AIDS) represents the modern pandemic that will continue to affect human history for at least decades. Other examples are cholera and influenza, which are capable of causing pandemics.32
In addition to widespread disease caused by epidemic spread of infections, some infectious diseases, because of their highly endemic prevalence in populations, continue to affect large segments of the world's population. These include enteric and respiratory infections, measles, malaria (which still causes 1 to 2 million deaths per year, especially on the African continent), and tuberculosis (which has become the number one killer in the world). Schistosomiasis is an important disease, affecting more than 200 million people worldwide. Furthermore, even the staggering mortality and morbidity of these tropical infectious diseases do not control populations but are associated with population overgrowth. This is true not only across the different countries of the world but also throughout the history of developed countries. Thus, the impact of these infections is not solely on the individual but, because of their highly endemic or epidemic occurrence, on populations. This has consequences on economic, political, and social functioning of entire societies.33
In an otherwise healthy and fully nourished person, a new infection is likely to be the only active infection in that person. In contrast, in regions where enteric and other infections are highly prevalent because of inadequate sanitation and poor socioeconomic conditions, adults and especially children may harbor several infections or be subject to repeated episodes of new enteric pathogens. Thus, the polyparasitism of multiple concurrent or recurrent infections adds a new dimension to the impact of acute infections, not often encountered in developed countries.
Moreover, the subclinical impact of a number of tropical infectious diseases is beginning to become apparent. Increasing data suggest that even "asymptomatic" giardial,34 cryptosporidial,35 and enteroaggregative E. coli36 infections may be very important in predisposing to malnutrition, thus reflecting a clinically important impact, even in the absence of overt clinical disease such as diarrhea. Likewise, chronic intestinal helminth infections also have a major impact on nutrition in those with already marginal nutrition. Anthelmintic therapy in these children, who lack symptomatic infections, has led to increases in growth, exercise tolerance, and scholastic performance.37,38
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