Why isn't swine flu spreading more than it is?

We've been hearing all kinds of stuff in the news about the swine flu pandemic, and how it's contagious and how people need to be quarantined if they have it. Er, sure, okay. But let's say, for example, I get it. I don't know I have it. I feel a bit crap, but like usual that doesn't stop me going to work because if you didn't go to work every day you felt a bit crap you'd never get anything done. But I digress... So, there I am on the tube. I'm not well, I'm breathing my swine flu germs all over many people while crammed in, while threading my way through the crowded underground stations and roads, while at work, while getting snacks and lunch, and while on the way home. Why don't all those people then get sick? All those terrorist studies say the best way to spread a virus is by releasing it on a tube train or at a hub airport. That's surely happened (it's around the world) but we're counting cases in hundreds not hundreds of thousands.

So what is it about this virus that we're not being told?