The little things are a much bigger worry.
Like you said, a T. Rex may be mean, but nothing that a .50 cal couldn't take care of in short order. Microbes on the other hand, are a totally different can of worms. It's gotten to the point now to where we can pretty much do anything we wish as far as many microbes are concerned. Their genetic code is very simple and makes it extremely easy to manipulate.
The most basic structure of life on this planet is now being altered by our technology. It's going to be very interesting to see where this leads.