Finally, there is a significant risk that an animal strain of flu couldmutate and become transmissible between humans potentially killing millions3.A mutated strain would potentially be more deadly, something that will belooked at later on in this essay. This conclusion can be made from the’Trade-off’ hypothesis’ that states, from an evolutionary aspect, pathogens areforced to make certain compromises8. The most notable of these isbetween transmission and virulence (how deadly the disease is), that in orderto spread more efficiently a disease must evolve a lesser virulence to killfewer hosts8. Thus one can see that it is less likely that anendemic disease would be responsible for an extinction level pandemic.
However,this theory has its flaws and is yet to be fully justified. For example in 1950a biological control agent, the myxoma virus, was released into Australia tokill rabbits- this disease attenuated (lost virulence) thus reinforcing thehypothesis. However, in 1995 the Rabbit HaemorrhagicDisease Virus, released for the same purpose, actually increased in virulencethrough time.The World Health Organization collects information on global deaths by International Classification of Disease (ICD) code categories. The following table lists the top infectious disease by number of deaths in 2002.
1993 data is included for comparison