Disease does not concern itself with political or geographical boundaries. The prevalence of a disease in one part of the world, therefore, is a constant threat to all other parts. For better or worse, the existence of many infectious diseases has been influenced by human alterations of local, regional and global ecosystems. For example, death rates from gastrointestinal diseases declined in industrial nations in the early 20th century in part because of the invention of the internal combustion engine. Cars replaced horses, meaning that disease-carrying flies had fewer breeding grounds as cars did not leave heaps of manure.

New vectors

Conversely, in the tropics and sub-tropics, dams built to store water for irrigation and hydroelectric power have introduced water-borne diseases such as Schistosomiasis to communities where they previously did not exist. Similarly, daycare centres are popular in this age of small families and working mothers. They are also an ideal settling for the transfer of infections among infants and toddlers, who are naturally affectionate so they touch, hug and kiss one another. But they have not yet been trained in personal hygiene. The common cold, diarrhoeal diseases and more serious infections can spread like wildfire and when taken home by the children, they infect parents and other children in the house. Again, air conditioning is widely used in hotels and hospitals in warm countries but dilapidated air conditioning units accumulate many kinds of moulds and microbes, including microorganisms responsible for a kind of dangerous pneumonia and some forms of meningitis.

International trade, which transports passengers and material consignments between countries, can also aid the infiltration of disease-carrying insects and pathogens to a new country. Dengue fever and malaria, for example, can be transmitted from one place where it is prevalent to another where it never existed. Besides, the green house effect, mainly due to fossil fuel combustion, is changing the world’s climate. For example, mosquitos that carry malaria, yellow fever, dengue and viral encephalitis are extending their range into temperate zones, higher altitudes tropical regions and large cities. By the middle of the 21st century, there may be several hundred million more cases of malaria each year as a result, many of them in regions now free of malaria. But not all ecological changes are caused by human activities. Blue green algae found in tropical pacific regions may harbour cholera vibrio, which can survive for lengthy periods in various forms in seawater.

In the days to come, the flight of migrant workers and landless peasants from rural areas in developing countries like ours also means that future generations of over half the world’s population will live in big cities. Many of them will live in slums, which favour the transmission of infectious diseases like tuberculosis, HIV/AIDS and Hepatitis B. Tuberculosis has always flourished in conditions of poverty, overcrowding, poor nutrition and illiteracy. HIV infections spread with prostitution and intravenous drug use.

New diseases

Finally, there have been several recent cases of new emerging diseases due to exposure to an unconventional infecting agent. Many kinds of new diseases have emerged due to consequences of human interference with natural processes. A major aspect of disease control relates to breaking the chain of transmission. This may mean changing some components of man’s environment to prevent the infective agent from a patient or carrier from entering the body of susceptible person.

Lastly, it goes without saying that the needs of the many must prevail over those of the few. Since health is influenced by a number of factors, such as adequate food, housing, basic sanitation, healthy life styles and protection against environmental hazards and communicable disease, the frontiers of health extend beyond the narrow limits of medical care. Healthcare is a public right and it should be maintained by blocking all possible factors that may arise during day-to-day behaviour.

Chaulagain is a retired public health officer