The last corner of the epidemiologic triangle is the environment. Disease transmission requires the passage of an infectious agent from one host to another, meaning that the pathogen must spend at least some time outside the host. The environment it encounters outside the host affects its ability to get from one individual to infect the next one before it dies. Malaria transmission, for example, can only persist where there are enough Anopheles mosquitoes, the obligatory vector for malaria parasites, to maintain transmission, such that each infection leads to at least one new infection. One of the most important environmental variables that affects our risk of infection is access to clean water and sanitation. In the modern era, developed countries have drastically reduced or even stopped transmission of most diarrheal diseases by breaking the link between waste water and drinking water, thereby blocking the fecal-oral route of infection used by many gastrointestinal pathogens. This graph shows the declining death rate from typhoid fever, caused by ingestion of Salmonella typhi bacteria in one U.S. city, as clean water was introduced in the early 20th century. Systematic studies have shown that although different cities introduced these sanitary measures at different times, the decline in infectious disease death rates coincided for each city with the introduction of clean water and sanitation. While the environment is clearly important for vector-borne and water-borne diseases, it can also affect the spread of directly transmitted diseases. These data from the 1930s show the proportion of white children in rural Alabama in the Southern U.S.A. who had been infected with diphtheria by various ages, depending on the level of crowding in their homes. You can see that individuals who lived in the most-crowded homes, shown in brown, were more likely at each age to have been infected with diphtheria, indicating that diphtheria infection risk is greater in more crowded living conditions. Hospitals are also environments that, unfortunately, are favorable to transmission of infections. This has been known at least since Ignaz Semmelweis introduced hand washing by medical personnel as an infection-control measure in a maternity hospital in Vienna in the 1840s, an intervention that reduced maternal mortality from childbed fever, an infection spread by doctors as they examined patients, often directly after performing autopsies. Semmelweis's famous intervention occurred before the germ theory of disease was well established, but now we understand that hospitals combine many factors favorable to the spread of infection. Immobilized, already ill patients who are vulnerable to new infections, high levels of antibiotic use that disturbs the normal bacteria living in and on a patient and increases susceptibility to infection, the frequent use of invasive procedures, such as catheters, that can bring bacteria through the skin, breaching our natural barriers, and medical and nursing personnel that can spread infection from one patient to another. Some infections, such as ventilator-associated pneumonia and catheter-associated infections, are limited to hospitals where such procedures are performed. Others may spread in any setting, but do so more efficiently in hospitals and other medical settings. MERS infection, for example, has spread from the animal reservoir to people and from person to person in many different settings, but the most extensive outbreaks have been in hospitals such as this early one in a dialysis unit in Saudi Arabia. Other environments are conducive to the spread of particular types of infections. Schools with large numbers of young children in close proximity promote the transmission of respiratory infections, and at younger ages, daycare centers can promote the spread of diarrheal disease. One study showed that university students in the U.K. experienced a three-and-a-half fold rise in the prevalence of carriage of the bacterium Neisseria meningitidis during the first week of term, from 7 percent to 24 percent. While carriage of the bacterium is not a problem in itself, it leads, in rare cases, to very severe bacterial meningitis. This rapid increase in prevalence, shows that universities are favorable places for the bacterium to spread. Further analysis showed that the major risk factors for being a carrier included kissing, going to nightclubs, and smoking, as well as living in mixed-sex residence halls. Many diseases show distinctive seasonal patterns driven by a combination of changes in the weather and changes in human behavior that respond to the changing season. Influenza is markedly seasonal, with strong winter peaks in temperate regions and more even distribution across the year in tropical settings. In temperate regions, variations in absolute humidity, the quantity of water vapor contained in a given volume of air, drive a part of this seasonal variation, with dry conditions favoring influenza transmission. This graph shows an experiment with guinea pigs in which the vapor pressure, a measure of absolute humidity, predicts flu transmission between guinea pigs housed in separate cages. Later work has shown that this explanation accounts for much of the winter seasonality in human flu in temperate countries. Another factor, though, is school terms. Flu's ability to transmit increases by some 15 to 20 percent when school is in session. Similar effects of school terms have been seen for other respiratory diseases. Another reason for disease seasonality is seasonal variation in the abundance of the vectors that transmit disease. Rain and warm temperatures promote mosquito population growth, and temperature strongly affects the ability of many vector-borne pathogens to mature within mosquito vectors. For this reason, many vector-borne infections, such as malaria and dengue, often show strong seasonal patterns. What are the implications of these environmental heterogeneities for disease control? It depends. In some cases, such as sanitation and clean water, we can make the environment less favorable for disease transmission while accruing other benefits. For this reason, clean water and sanitation are, or should be, high on the development and public-health agenda in resource-poor settings. In other cases, the environment where transmission takes place, is an environment with intrinsic value. A hospital, school, or university. In such cases, avoiding the environment is not a desirable option, but improving infection control and hygiene within the environment can be highly effective, as in the control of SARS, and more recently, the control of MERS. In extreme cases, hospital units or schools have been closed temporarily to stop transmission when infection-control measures don’t work. And then there are environmental drivers we can't do much about, such as the weather. Here, the best we can do is to use the relative predictability of outbreaks, for example, of flu in the winter, to plan vaccination campaigns and other responses.
