How does your immune system attack bacteria, viruses, parasites, etc.?

[porchrat]

For how long can bacteria and viruses live outside their host?

Depends on the pathogen. Usually minutes or less. Though keep in mind that some are able to enter a spore-like stage that means they can in some cases survive indefinitely.

What constitutes a hospitable environment for them? What simples measures can one take to make things less hospitable for them?

Again it varies from pathogen to pathogen. Heat is usually a great way to kill bacteria and viruses. Most bacteria and viruses depend on a very narrow range of temperatures in order to survive. Alter that range by more than a degree or 2 and they tend to expire.

Altering the environment is also a good one. Changes in pH or introduction of some other compound not usually found in their host environment. Things like vast quantities of alcohol for example.

To my knowledge, viruses don't survive well outside the human body, but some bacteria can survive up to a few days on things like door handles.

As mentioned above it is usually minutes or less. Parasites are worse though. A lot of them are able to survive indefinitely. You find their egg structures on money, handrails, all over the place.

Putting food in fridges and freezers won't necessarily kill bacteria on the food either.

Just tends to slow them down a bit. Remember that a lot of the rotting process you see in food is facilitated by bacteria.

Some bacteria (e.g. Helicobacter pylori) can survive stomach acid.

Stomach is not your only defence. It isn't even your first line of defence. The mucosal lining of your mouth and the flora found in your mouth are your first line of defence.