We need to institutionalize a sentinel system to identify and diagnose potential pandemics. A truism from the medical profession is that one does not do the work to diagnose something that one is not going to treat. Thus we also need an effective global system to react quickly to control and end potential pandemics quickly and completely. The current Ebola epidemic demonstrates that the world now has neither an adequate sentinel system nor an adequate global response capacity.
It seems very likely that humans evolved in small bands that were relatively free of communicable diseases. Those bands probably did not interact frequently nor closely, so that even if a disease was present in one band it was unlikely to spread to others. Humans lived in the Western Hemisphere for more than 10,000 years with few communicable diseases - the original bands strong enough to make their way to the Americas must have been healthy, or must have left their communicable disease burdens behind. Communicable diseases became more common in Asia, Europe and Africa as their populations increased, as towns and cities developed, and as diseases could travel long distances more easily.
Pandemics are big epidemics -- one that affect world population. History is marked by a number of known pandemics. Some that come to mind are the Black Death of the middle ages, the decimating series of plagues that hit the American Indians as Europeans invaded the Americas and the Syphilis epidemic that was the return gift of the Americas to the old world, the Spanish flu at the time of World War I (that killed some 50 to 100 million people worldwide), and HIV/AIDS that has already killed 39 million people.
Perhaps worse still are the communicable diseases that become endemic. Even diseases that result in lifelong immunity (such as measles) can remain endemic in a population if enough susceptible new entrants (babies, immigrants) continue to arrive to keep the germs in circulation. Flu is with us all the time in part because winter flu season alternates from northern to southern hemispheres and back, but also because the virus has the ability to mutate (often in another species) in such a way that it evades the immunity built in humans in previous flu seasons and epidemics; we expect another flu pandemic, but don't know when it will arrive. Malaria is endemic in large parts of the world, with a heavy burden of disease and death (WHO vowed to eradicate malaria but failed).
So where do communicable diseases come from that have the potential to cause pandemics? They They come from insects, animals or birds, or even plants. How does this happen? People come into contact with something carrying the microbe, get sick, and begin the process. Of course, if there are more people, it is more likely that one of them comes into contact with new germ. If there are more chickens or pigs, it is more likely that a person comes into contact with a chicken or pig carrying a new germ. If the environment changes (due to climate change, desertification, deforestation, etc.) then insects and animals may move into new contact with man sharing their germs.
Usually a microbe arriving in a new species it poorly adapted to that species and often a relatively easy target for the immune system of the new host to eliminate. Occasionally, however, microbes will escape destruction long enough to evolve to resist the immune system of the new host and infect others of that species. The more often a species of microbe infects a member of the new host species, the higher the possibility that it will become an effective disease agent.
The likelihood that a disease will move from one individual varies greatly. Really amazing adaptations have occurred, such as the microbe that infects mice, leading them to become less afraid of cats, and then infecting the cats that eat the mice. In general, the greater the contact between hosts, the more likely the disease will successfully make the jump from one host to another. Thus crowded cities are more likely to see transmission of human diseases than disbursed rural areas; the modern practices for breeding livestock in large concentrations similarly provide rich areas for transmission of their diseases. The more hosts are involved in an epidemic, the more likely that the microbial cause of the disease will evolve to become still more contagious.
There are scores of infectious diseases known to infect humans. Some have been around for thousands of years, some have been little studied having been identified in only limited and localized outbreaks. I don't see any way to predict the probability of emergence of a new pandemic infection, but it seems to me that they occur more than once a century -- and they may occur more often as populations increase and the environment changes more rapidly. Can we afford even a one in a hundred chance of another pandemic of the magnitude of the HIV/AIDS or Spanish Flu epidemic in a decade? If not, then we should institutionalize the global system to detect and stop them before they grow out of our ability to control them.
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