Homo sapiens history is millions of years. Then why we do not have contagious disease from millions of years ago. All the harshest contagious disease happened in couple decades. And they likely to stay in our specie's gene pool. such as HIV. How come it did not exist from millions of years ago? If it’s transmitted from monkey. We ate more monkey millions years ago. It is very mysterious. Since some disease like HIV will be likely to stay in our specie in the future but non-have stayed since millions years ago. And we did not even have medication for cure millions years ago. Isn’t it interesting? Can be new disease still happen to humans all the sudden? And why does it happen? If HIV existed in millions of years ago wouldn't we all be extinct? Or actually medication to prolong life is bad effect to the society as whole? Because its cause more people to be contaminated and not die naturally. Anybody know the answer? 

 

I'd say that a large part of the reason we don't have diseases from millions of years ago(and we don't really know that we don't, only that such diseases don't have any great effect on us due to evolving immune systems) is because at a time when humans and protohumans had no medical understanding, the more aggressive diseases killed themselves off by killing off all of the affected people.

In addition to what Ideal said... humans were also more isolated from each other.  If a horrible virus sprung up in a community, it would not be easily transmitted to another community....

Nobody was globe trotting from the US to get married in Europe, back and forth in a confined airplane with recycled air for example.

 Which brings up a third point...studies have shown that cracking a window in the office cuts down on the severity of flu outbreaks...having to 'crack a window' to let in a little ventilation was not a concern a few decades back.

black plague not enough for ya?

It has been shown that much of the DNA in our cells is a result of retroviruses.  To boot, HIV is a retrovirus.  A retrovirus invades the cell and becomes part of an individual's genome, and generations pass them down through vertical gene transfer.  The way a person picks up a virus or retrovirus is through horizontal gene transfer.  HGT is, simply put, the process of individual contracting DNA from it's environment. 

So, we have ancient virus' within all of us.

Here is a link describing retrovirus' and our genome: http://www.pubmedcentral.nih.gov/articlerender.fcgi?&pubmedid=16611924

Here is a link describing vertical and     :

http://www.physorg.com/news114185292.html

I hope this finds you curious!