I agree that the concentration of people can make for easier spreading and it's definitely not healthier than smaller towns, but don't underestimate how much damage can be done without this civilization thing we've got going on.
On the subject of the thread:
I think it's not a crazy idea to say vaccinations could be mandatory. Just like paying taxes and sticking to traffic rules, this could be a mandatory thing. I would personally not be opposed to it, but I know there are people that don't trust the government enough to allow something like this. In theory a system like this could be abused by an ill-meaning government. I don't think such a thing is likely, but I can see how this would be a problem for some people.