What's the difference?
Here’s one that is not listed in The Longman Language Activator, a dictionary which effectively comes up with alternative words and their meanings when you want to communicate something in writing, but are unsure if the choice of word is right in the context.
First, an organism is a very, very small – so small you need a microscope – animal or plant. You can’t see them with your own eyes. These organisms can be both harmless and harmful.
Next, a germ is a very small organism. The word itself, however, is a general term often used to describe anything that causes illness, and is not very often used by doctors and scientists. However, this particular very small organism can be harmful, in that it causes disease. So when someone’s got a cold and they sneeze, you may breathe in their germs and catch a cold. The chemical chlorine is used to kill germs, and that’s why it’s added to swimming pools. Many household cleaners also contain a lot of chemicals to do the same job.
Now bacteria are (or should we say bacterium or even bacterias? click here for the answer to that puzzle) very small organisms that can be found anywhere. Not all of them are harmful – they are used in yoghurt and cheese, for example - but some can cause disease. Scientists often prefer to use this word instead of germ.
Finally, a virus is a kind of germ, but we can't really call it a bacteria because, medically, they are not really living as such and so do not reproduce themselves. However, they can deceive our existing cells, making them believe they're okay, and it is our cells that end up reproducing the virus and so cause disease and spread from one living thing to another. In this way, this is precisely how a computer virus works: it gets into a system, fools the computer and ends up altering and destroying information. If you find it in your mail, your computer will be fooled. If you forward the email to somebody else, then they're going to get it too,
So in summary: An organism is a very, very, small living thing. A germ is an organism that causes illness; bacteria(s) is/are (your choice) organisms that can be both helpful and harmful and is a more scientific word than germ; a virus is a non-living organism that is spread by the host it infects.