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This is my first post here, so excuse me for its simplicity.

Viruses can infiltrate a cell, overtake it and multiply. It has projecting fibers whose ends are shaped as kind of a "key" to a mobile sentry on the cells membrane so it can be permitted access into the cell. When it enters the endosome of a cell, a protein pump blows an acidic gas onto the virus to break it down, thinking it's a special nutrient. But the virus expects this of course, and when the protecting fibers break off (and so too the body of the virus) proteins are released that rips apart the skin of the endosome so the virus can escape.

These are just two of many things the virus does to compromise a cell. Its entire existence is for the sole purpose for infecting a cell, and it seems to know how to do that perfectly. How did the virus evolve to know how to do things like this?

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Do you have a specific virus in mind? This is incredibly broad, and we'll honestly never know; we weren't there to study most evolution. The short answer is "over many years." –  Amory Sep 13 '13 at 22:46
    
@Amory Lets just say the common cold. –  0x499602D2 Sep 13 '13 at 22:55
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It's a numbers game. Viruses can produce thousands, millions, even billions of copies every day, and only one of them needs to be slightly better to get ahead. Rhinoviruses make roughly one error every time the genome is replicated; over many, many replications a single change can arise giving an advantageous effect. As another example, it's been said that HIV mutates every single spot on its genome every single day. Evolution is simply a waiting game.

This article is a pretty cool write up detailing the process. It says exactly what I said: large numbers, high mutation rate. Besides, the virus only has to take advantage of cellular processes, not make new ones every time. That's much more efficient. It also details some specific changes that explain some differences between similar viruses. It's amazing, but a single base change can mean a single protein is different which can mean a different receptor is used for binding, which can make all the difference.

And there's this cool quote:

[T]here may well be more viruses in a single common cold infection than there have been primates in the entire history of life on Earth.

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Scientists are unsure, but it seems that viruses have been around a very long time, and have been specific to every domain of life and possibly even may have infected the last common ancestor. It isn't impossible to imagine a co-evolving arms race that makes both viruses and cells as complex as they are today.

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