Why can bacteria or viruses that are new to our body be so dangerous?

One of the ways your immune system can go haywire is by not recognizing a potential threat. For example, when you're immunized with mumps early in your life, your body gets the information about what the mumps virus look like. Your immune system then stores it in its memory data bank, so if you're ever infected with the mumps virus again, it would pull out the file on mumps, immediately recognize those cells as intruders, and send out the right defense in order to thwart an attack-usually without you ever being the wiser.

However, when your immune system doesn't know what a potential intruder looks like, that's when you have problems, because your body has no data-no record of criminal activity of sorts-and no record of what the virus or bacteria looks like. So your immune system has no ability to respond to it. (You start to lose those precious criminal files as your immune system gets older, so some old enemies get a chance to infect you again.) In the case of your typical flu virus, your body knows parts of it (it mutates every year, it seems, so you only know a part of the newer strain unless you've received a current flu vaccination) and is able to fight it off.

But when a virus is completely new to your body-as in the case of the SARS virus-it doesn't display markers that indicate that it's foreign to your body. Without that prior file, your immune system doesn't respond, leaving the virus to rummage through your things and destroy whatever it chooses to, whether it's part of your nervous system, respiratory system, or something else. This is why microbes that are new to us-or at least seem new to our immune system-can be so deadly.

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