The Consequences of Viruses
Viruses have had an enormous impact on our history. They are responsible for a wide range of diseases, from the mostly harmless common cold to deadly ebola. Reaching far back into prehistory, viruses have afflicted whole civilizations - and as we have evolved, they have evolved along with us. Ironically, many of our advances, such as modern transportation, cities, and sanitation, have created an ideal environment for viruses, allowing for rapid distribution of new viruses around the globe, large, concentrated populations, and lowered immune defenses due to improved sanitary conditions.
The infamous polio epidemic of the 1950's, for example, was a disease born from progress. Before the advent of modern sanitation, the polio virus was everywhere, almost everyone was or had been infected, and almost everyone was thereby immune. Infants were protected from it by their mother's immunity, passed on to them when they were still in the womb, and repeated exposures throughout childhood - resulting from open sewage and the contamination of water and food - guaranteed that they retained this immunity. When they did occur, polio infections were rarely dangerous or widespread.
Modern sanitation changed all this. Sewers, safer food handling, and water purification eliminated the virus from everyday conditions, and natural immunities gradually degraded and were lost. New generations of children did not become immune to the virus. When polio finally struck, it found populations (particularly in the US and Europe) with no natural immunity even in late childhood - and the effects were devastating.
While viruses have all left their mark on our history and evolution, some - the retroviruses, for example - have even left their mark on our DNA. The human genome is filled with remnants of retrovirus genes, copied into the genes of our ancient ancestors, and passed down to us through thousands of generations.
Viruses have given us more helpful gifts as well, teaching modern science about the operation of cells and the immune system. The retroviruses have once again left the most indelible mark - for from them scientists have learned the lessons of genetic engineering. Retroviruses taught us how to splice genes and alter the code of life, just as they do themselves in their reproductive cycle.
Over the past eighty years, vaccines have been developed against many of our worst diseases; yet in many cases our efforts have only promoted virus evolution, selecting for the worst viruses and eliminating the harmless ones. As diseases are eliminated, new viruses emerge to fill the void.
Viruses will clearly be with us for a long time to come.
Many viruses still elude our understanding, and it is these which pose the greatest threat. AIDS has captured our attention for the past two decades, and enormous progress has been made towards understanding its reproduction and finding a way to stop it. Hepatitis C is perhaps the next challenge, representing what may be one of the greatest public health threats we will face in the coming century.