I'm wondering how can very uncommon diseases remain in a relatively stable equilibrium. For example, if it started to spread, the spread could accelerate and become an epidemic. However, if it doesn't spread, why doesn't it wither away?

As I understand, the ebola for example, is highly contagious, nearly always fatal, and kills quickly. So quickly, hat the target has not much time to pass it on to others. If it spread just a little better, it would become an epidemic, just a little slower and it would disappear. How is it possible, that its spread did not change significantly in the last couple of decades? It kills a few dozen, maybe a few hundred people per year, these numbers remain largely unchanged.

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    $\begingroup$ They do not maintain an equilibrium. They show up, kill a few people and are gone again. Something stable would be like the measles which are present in humans (and only there) and take the chance to cause local epidemies. And we are lucky that ebola kills so fast and is not better transmittable. This would end in a big catastrophe. A very interesting book on this topic is "Spillover" by David Quammen. $\endgroup$ – Chris Mar 24 '14 at 14:42
  • $\begingroup$ I did not mean that they intentionally maintain an equilibrium. I was just meaning, that it stays a it was decades ago. Not growing, not shrinking. $\endgroup$ – vsz Mar 24 '14 at 15:12
  • $\begingroup$ What do you mean by "it stays as it was"? That there are no mutations? That it is not moving to other hosts? $\endgroup$ – Chris Mar 24 '14 at 15:53
  • $\begingroup$ No, the number of people infected per year. $\endgroup$ – vsz Mar 24 '14 at 16:32
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    $\begingroup$ Why should it? This virus exists in its reservoir hosts (most likely bats). From there is breaks out every now and then, when people cross the way of this host or probably eat them. This virus kills its host quite efficiently, so it doesn't stay in the human population (additionally humans do all to prevent it staying there). I really recommend the book by David Quammen, although is popular science. It is interesting to read on this topic (not only Ebola). $\endgroup$ – Chris Mar 24 '14 at 19:07

Viruses and other disease causing organisms are usually in an ecological steady state in different organism, called a vector. When they start to infect another organism similar enough that they can reproduce, they may turn out to be more virulent or fatal than is optimal for the organism.

The classic example is the case of influenza outbreaks, the flu virus is causing mild infections in other animals such as birds or pigs and then a human being catches it and it may be serious or even fatal. This is classically because the viruses lytic cycle (where it reproduces and then kills host cells) is not tuned to the equilibrium ideal for viruses in the new human environment, which is not fatal which gives the virus the best chance of reproduction and retransmission.

In the case of Ebola and Marburg viruses the vector is not known but there is speculation and research going on. So in the extremely rare handful of events where these hemorrhagic fevers have been caught by people they fatal.

Its the non-equilibrium case where the virus is not adjusted to human hosts and adjusted for its vector population where it is usually most dangerous.

If these viruses were allowed to persist they would theoretically come to the point where it wasn't fatal. The problem being that people would die and there is always the risk that the virus would burn through all possible human hosts before it adjusts...

I want to say that this is all driven by evolutionary forces. If the environment encourages pathogenic characteristics then human pathogens can persist forever. Flesh eating bacteria and MRSA and other antibiotic resistant bacteria have found a competitive environment where causing disease and death are the only way to go - hospitals with antibiotics. In the case of Ebola/Marburg, there may be a disease reservoir of monkeys acting as a pool for the high fatality strain. Anything can happen with the right geography and dynamics.

If you've been thinking to yourself, 'what are the odds that a virus or bacterium would be extra virulent versus not infectious at all?', you are right. One should also note that fatal outbreaks are rare and exceptional. There are clearly plenty of cases where viruses or pathogens from other organisms can't get any traction at all in humans. We are practically swimming in viruses and bacteria all the time.

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    $\begingroup$ They mostly getting less dangerous over time, but not necessisarily harmless. The Myoxomavirus, which was imported to kill rabbits, still kills about 70% of all infected animals, and this is more than 30 years after it was set free. In the beginning the rate was around 99,6%, but 70% is still far away from being nice. The example is from the Quammen book, the data has been published [here](Biological Control, as Exemplified by Smallpox Eradication and Myxomatosis). $\endgroup$ – Chris Mar 24 '14 at 19:56
  • $\begingroup$ Adding a comment for @PlaysDice - recommending this book that explores emerging threats from reservoirs of infectious biotica. amazon.com/dp/0393066800/?tag=stackoverfl08-20 $\endgroup$ – shigeta Mar 25 '14 at 11:09

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