I'm struggling to describe a virus-like outbreak, but this is caused by something that does not act, look, or is really even a virus. how do i convey that its nothing ever seen before, but is comparable to a virus?
by describing it the way you just did! fyi, all communicable/infectious diseases are not viruses... tb, for instance, is caused by bacteria... and there are other causes, as well... do some googling and study up on what you want to write about, if you want it to make sense...
Jeff Carlson is an international best selling author whose basic story is about nanobots that become killers. They spread like a virus and are very difficult to fight. His treatment of this disease-technology is very virus-like. You might want to drop a few $ (or buy a used copy) and read his book as an excellent example of what you're describing. In fact, he offers the first couple chapters in each book as a free download. That might be enough for your purposes. http://www.jverse.com/index.html I have his first two books (signed) and will be buying the third shortly.
Simply have a few scientists put tissue samples (bloods samples, brain samples, etc.) under a microscope and run a few tests.
Watch Numb3rs episodes. No, really, they do seem to pour a lot of viruses into their research. It's a fairly acurate description, but what causes the viris? Nothing? Then write that. Does it just appear? Write that. Be sure to name it!
Perhaps some sort of environmental toxin that is spreading along a water way, giving the illusion that a virus is spreading along the water way.
