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It seems like every disease we ever hear about is something that's been around since ancient times, since thousands of years ago. Of course new diseases were catalogued over the course of the past couple hundred years, but they weren't actually new diseases-- they just hadn't been previously classified/identified.

Given that (in my understanding) viruses and microbial lifeforms can evolve and change considerably more rapidly than higher order organisms, why don't we often/ever hear about completely new diseases that have never been seen before? Diseases that may have only just been born.

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You might find this interesting: royalsociety.org/events/2006/plagues-parasites/… –  Chinmay Kanchi Mar 27 at 16:37
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Evolution doesn't really work like that, everything is a slightly different version of what we had last year, it's only on the very long term that you can see major change. New flu arrives every year, but we still call it flu because it's only a little different –  Richard Tingle Mar 27 at 17:59
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HIV anyone? It's new in the sense that it only came to infect humans recently. –  monoceres Mar 27 at 18:58
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The terms about we are speaking are relatively long. There are a number of pathogens (mostly viruses) which are relatively new. Among them are SARS and MERS, HIV, Ebola, Marburg, Hendra and so on. These are mostly viruses which spill over from their reservoir hosts into humans. There is an excellent book on that topic, which I can really recommend. It is "Spillover" by David Quammen. –  Chris Mar 27 at 19:14
    
I think the answer is completely given in the comments. One could group these comments to make an answer and add some link to seasonal flu or to new pathogens. –  Remi.b Mar 28 at 7:37

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The first thing to note is that nothing is truly new, everything is a slightly different version of what existed previously, it's only on the very long term that you can see major change. Mutations occur continuously and new variants arise often. For example you need a new flu shot every year due to the high mutation rate of the influenza virus but we still call it flu because it's only a little different.

While disease causing organisms evolve at a relatively slow rate new diseases can appear suddenly when the effect on humans suddenly changes; the most obvious example of which is when a disease changes host species. For example the sudden appearance of HIV when Simian immunodeficiency virus (SIV) "jumped species" to infect human, most likely through the consumption and processing of Primate meat. While HIV appears new it is again ancient in the sense that it has caused a similar condition to HIV/AIDS in many African non-human primates for a significant period.

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