Density of population does not of itself determine the ease with which infection spreads through a population. Problems tend to arise primarily when populations become so dense as to cause overcrowding. Overcrowding is often associated with decreases in quality of living conditions and sanitation, and hence the rate of agent transmission is typically very high in such areas. Thus, overcrowded cities or densely populated areas of cities can potentially serve as breeding grounds for infectious agents, which may facilitate their evolution, particularly in the case of viruses and bacteria. Rapid cycling between humans and other hosts, such as rats or mice, can result in the emergence of new strains capable of causing serious disease.

Social habits

The vampire bats of Brazil, which transmit paralytic rabies, bite cattle but not ranchers, presumably because ranchers are few but cattle are plentiful on the plains of Brazil. Bat-transmitted rabies, however, does occur in humans in Trinidad, where herdsmen sleep in shacks near their animals. The mechanism of infection is the same in Brazil and Trinidad, but the difference in social habits affects the incidence of the disease.

During the early 20th century in Malta, goats were milked at the customers’ doors, and a Brucella species in the milk caused a disease that was common enough to be called Malta fever. When the pasteurization of milk became compulsory, Malta fever almost disappeared from the island. (It continued to occur in rural areas where people still drank their milk raw and were in daily contact with their infected animals.)

Important alterations in environment also occur when children in a modern community first go to school. Colds, coughs, sore throats, and swollen neck glands can occur one after the other. In a nursery school, with young children whose hygiene habits are undeveloped, outbreaks of dysentery and other bowel infections may occur, and among children who take their midday meal at school, foodborne infection caused by a breakdown in hygiene can sweep through entire classes of students. These are dangers against which the children are protected to some extent at home but against which they have no defense when they move to the school environment.

Changing food habits among the general population also affect the environment for humans and microbes. Meals served in restaurants, for example, offer a greater danger of food poisoning if the standard of hygiene for food preparation is flawed. The purchase and preparation of poultry—which is often heavily infected with Salmonella—present a particular danger. If chickens are bought fresh from a farm or shop and cooked in an oven at home, food poisoning from eating them is rare. If poultry is purchased while it is deep-frozen and then not fully thawed before it is cooked, there is a good chance that insufficient heat penetration will allow the Salmonella—which thrive in the cold—to survive in the meat’s center and infect the people who eat it.

Temperature and humidity

At a social gathering, the human density per square yard may be much greater than in any home, and humidity and temperature may rise to levels uncomfortable for humans but ideal for microbes. Virus-containing droplets pass easily from one person to another, and an outbreak of the common cold may result.

In contrast, members of scientific expeditions have spent whole winters in the Arctic or Antarctic without any respiratory illness, only to catch severe colds upon the arrival of a supply ship in the early summer. This is because viruses, not cold temperatures, cause colds. During polar expeditions, the members rapidly develop immunity to the viruses they bring with them, and, throughout the long winter, they encounter no new ones. Their colds in the summer are caused by viruses imported by the crew of the supply ship. When the members of the expedition return on the ship to temperate zones, they again come down with colds, this time caught from friends and relatives who have spent the winter at home.

Movement

Movement into a new environment often is followed by an outbreak of infectious disease. On pilgrimages and in wars, improvised feeding and sanitation lead to outbreaks of such intestinal infections as dysentery, cholera, and typhoid fever, and sometimes more have died in war from these diseases than have been killed in the fighting.

People entering isolated communities may carry a disease such as measles with them, and the disease may then spread with astonishing rapidity and often with enhanced virulence. A traveler from Copenhagen carried measles virus with him to the Faroe Islands in 1846, and 6,000 of the 8,000 inhabitants caught the disease. Most of those who escaped were old enough to have acquired immunity during a measles outbreak 65 years earlier. In Fiji a disastrous epidemic of measles in 1875 killed one-fourth of the population. In these cases, the change of environment favored the virus. Nearly every person in such “virgin” populations is susceptible to infection, so that a virus can multiply and spread unhindered. In a modern city population, by contrast, measles virus mainly affects susceptible young children. When it has run through them, the epidemic must die down because of a lack of susceptible people, and the virus does not spread again until a new generation of children is on hand. With the use of measles vaccine, the supply of susceptible young children is reduced, and the virus cannot spread and multiply and must die out.

An innocent change in environment such as that experienced during camping can lead to infection if it brings a person into contact with sources of infection that are absent at home. Picnicking in a wood, a person may be bitten by a tick carrying the virus of one of several forms of encephalitis; as he swims in a canal or river, his skin may be penetrated by the organisms that cause leptospirosis. He may come upon some watercress growing wild in the damp corner of a field and may swallow with the cress almost invisible specks of life that will grow into liver flukes in his body, giving him fascioliasis, an illness that is common in cattle and sheep but that can spread to humans when circumstances are in its favor.