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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 '14 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 '14 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 '14 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 '14 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 '14 at 7:37

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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New diseases appear all the time. Some of them have been around for ages but are detected due to improved surveillance, while others appear due to shifts in host. Others are detected because they spread geographically into a region where they are either more conspicuous or just more highly reported/investigated, or because new hosts are introduced into their original range.

For example, in the last few years alone both the snappily-titled arthrogryposis hydranencephaly syndrome (caused by Schmallenberg virus, a previously unknown bunyavirus) and bovine neonatal pancytopenia (aka 'bleeding calf syndrome') have been discovered. Both were first detected in Europe, where veterinary surveillance is among the most intense in the world; Schmallenberg was detected because of syndromic surveillance in use in NL and DE. When bluetongue first emerged in Europe a slight difference in primer sets used in Switzerland detected a novel related virus that was called Toggenberg orbivirus (later classified as a new serotype of bluetongue virus). This may have been circulating in Europe undetected for many years and it's only by chance that it was detectable by the Swiss primer set (although it doesn't appear to cause clinical disease).

Incidentally, bluetongue and two other high-impact livestock diseases originating in sub-Saharan Africa (African swine fever and African horse sickness) appeared at the end of the C19th when European settlers introduced 'improved' breeds of livestock into the region; before that the pathogens were circulating subclinically in the natural hosts (warthogs for ASF virus, zebra for AHS virus and various ruminants for BT virus).

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