I think it's key that people develop a basic knowledge of food hygiene and pasteurisation techniques when they start cooking sous vide. We're used to easy and quick pasteurisation at very high temperatures, and frankly, not thinking about it very much.
No-one would leave meat out in hot weather for 24 hours, sealed or otherwise, as they'd intuitively realise that it would only lead to incubation of bacteria. It's important to know when the heat is sufficient that (with enough time), bacteria is reduced rather than multiplying.
You don't have to pasteurise every piece of protein you cook, but if you cook below 130F, the cook time needs to be short enough that you're not incubating bacteria for hours on end.
I found Douglas Baldwin's site and book an excellent place to start...I'm sure there are a lot of other resources (including this site) available, but this link is somewhere to start: http://www.douglasbaldwin.com/sous-vide.html#Safety