In the 14th century, bubonic plague–The Black Death–swept Europe, killing 25 million people, one quarter of the population. In the 18th century alone, smallpox killed 60 million people worldwide. Measles kills 900,000 people annually, mostly in developing countries. Respiratory infections such as influenza kill up to two million people annually. Intestinal infections such as cholera killed five million people in 1980–10 a minute. Worldwide, it’s estimated 10 million people are infected with AIDS.
All right, so it’s not one of the most cheerful opening paragraphs ever seen in the annals of column writing–it still serves to introduce my topic this week, which is infectious disease.
It seems like every year we hear of some new, previously unknown disease. In 1976 it was Legionnaires’ disease, a severe form of pneumonia which infected 182 delegates to the American Legion Convention in Philadelphia, killing 29 of them. Also in the ’70s appeared Lyme disease, named after Lyme, Connecticut, where the disease’s unusual symptoms were first identified. And, of course, in 1981, AIDS was first identified.
How can a new disease just appear out of nowhere? The answer is, in most cases, they don’t–it just seems that way.
Legionnaire’s disease, for example, is caused by a bacterium called Legionella pneumophila, which is common in cooling towers, lakes and other bodies of water. It only causes disease when certain conditions are met . Unfortunately, bacteriologists still aren’t sure what those conditions are.
Lyme disease is spread by the bite of ticks infected with a corkscrew-shaped bacterium called Borrelia burgdorferi. The particular species of tick, Ixodes dammini, feeds on deer. Both the bacterium and the tick have been around a long time–museum specimens of ticks collected in the 1940s have been shown to be infected with the bacterium. The disease has become more of a threat because the number of ticks has increased, due in large part to a burgeoning deer population, in turn due to the re-growth of felled forests in many regions of the northeastern United States. Last year 7,000 new cases of Lyme disease were reported.
And then there’s AIDS. It, too, has been around for a lot longer than most people realize. In fact, examination of the preserved tissues of a young sailor who died in Manchester, England, in 1959 proved that he was suffering from AIDS.
It’s generally felt that the AIDS virus has existed for centuries in African monkeys and apes, but that only sometime this century did the virus make the leap from simian to human, either through a mutation or simply because, as the population exploded in Africa, more people came in contact with monkeys.
Until AIDS came along, a lot of people were feeling pretty complacent about our relative immunity to disease in this modern day and age. After all, smallpox, one of the great killers of the past, has been eliminated worldwide. Another of the deadly diseases of history, bubonic plague, can be successfully treated with antibiotics. There are vaccines for polio, measles, mumps, various strains of influenza, and many other diseases. Better living conditions and hygiene, in the developed world at least, have made relative pussycats out of previously tigerish diseases like measles. Some people dared to dream of a disease-free world.
Trouble is, new diseases keep springing up. Often they already exist in some isolated area and make their appearance when that area is opened up to the outside world for the first time–for example, by the building of a road into a previously untracked stretch of rain forest. As well, viruses and bacteria keep mutating in the great on-going experiment of natural selection–and we never know when one of those mutations will prove wildly successful from the germ’s point of view, by allowing it to make use of that wonderful breeding ground called the human body, of which there are so many billions available.
The modern way of life is ideal for the rapid spread of any new, virulent disease, as well. Millions of people crowd into cities with populations concentrated in greater numbers than the world has ever known–and many of them live in poor sanitary conditions. Airplanes take people around the world in hours. Insects which carry disease can hitch a ride with a load of fruit bound for another continent.
Epidemiologists point out that isolated outbreaks of deadly, exotic diseases occur all the time. There’s a fatal hemorrhagic fever that has killed many children in various outbreaks in rural Brazil, apparently caused by a slight mutation to a common bacterium that causes conjunctivitis. There are diseases with names like Marburg fever, Rocio encephalitis and Ebola fever that have appeared and disappeared for no apparent reason–but there’s no guarantee they’re gone for good.
Still, lest I sound too gloomy, we are in better shape than our Middle-Ages ancestors. Science has given us the knowledge of how diseases are spread, of what to look for. If it cannot always provide a cure, as with AIDS, it can tell us what to do to limit our risk of infection. And even AIDS is beginning to yield to the ongoing scientific onslaught. There are new drugs showing promise and the possibility of a vaccine no longer seems so remote.
The threat of disease will probably always be with us, no matter how advanced our science becomes. But every increase in knowledge about the human body and the microorganisms that infect it reduces that threat . In the meantime our greatest defenses against new diseases will continue to be good hygiene, clean water, and decent living conditions.
It’s a shame that, in too many parts of the world, we can’t seem to find a way to provide them.