Since everyone is talking about viruses and the flu, I thought I would throw in my two cents worth and let's face it, these days it isn't worth much. Also be advised, I am approaching this topic from a natural history point of view, not a medical one.
Colds and influenza are both caused by viruses. Viruses are tiny little things, so small you can't even see them with the naked eye. You could breathe one in through your mouth or nose and not even know it. Or you could touch your face or rub your eye with an unwashed hand and get infected that way. Some viruses can make you sick, but many also help you.
So, what exactly are they, where do they come from and how long have, they been on the planet, bothering humans and countless other critters? To start with they are a part of the wonderful and mysterious world of microbiology which is a catch-all phrase for all sorts of little things that are alive but far too small to see. Sometimes people confuse viruses for bacteria, but they are completely different species just both very small. It would be a little like comparing elephants to ants. Yes, they are both critters but that's about it.
Bacteria are a single cell organism and they are probably one of the oldest organisms on the planet, along with algae. According to the geological record they even managed to create sizable reefs on the oceans floor back in the Precambrian era, probably a couple billion years ago. The warm oceans of the day were the proverbial primordial soup, we often hear of. Full of all sorts of little micro-organisms but since the bacteria and algae, both created reefs, we have a geological record of them. Then along came viruses to feed on the other microorganisms. Well sort of feed, which we will talk about later.
Both bacteria and viruses are very small. Think about this, they are so small, they can float around in the air and they can be found in just about every living thing everywhere on the planet. Some people speculated that such things existed, but it wasn't until humans invented very powerful microscopes, that people first saw them in 1670.
So just how small are they. Well you would need to magnify them a hundred times, just to see the largest bacteria and to see them clearly you would need to magnify them 400 times. Think about that. Imagine if you suddenly became 400 times bigger then you are now. If you looked down at your feet, there would be a tiny little you, that you probably wouldn't even see, or notice was there. Viruses are a whole lot smaller and to see them you would need a very expensive electron microscope, that can magnify things a thousand times or more. So, you really are comparing elephants to ants and they are separate species. Virus are tiny and are a slimy little chain of nucleic acid which can't survive long on their own and need a host body to live in. So, viruses live inside of animals, birds, reptiles, plants and even attach onto bacteria.
Like all living organisms they have several basic functions. To survive, to take in nutrients, to reproduce and to expand their range, as much as possible, to insure their species survival. Since they have been around a lot longer than multi-cellular organism, like us, they are pretty good at it and have developed all sorts of survival techniques.
So how do these little things reproduce. The bacteria like most single cell organisms, grow bigger and starts to replicate everything in the cell and then they divide in two. So, one bacterium becomes two, then four, then eight until they number is in the gazillions. Viruses take a whole different approach. One which is a little scary, amazing and leaves one stunned, how does something so small manage this. They attach onto a cell in their host. Then they start to rapidly make replicas of themselves in that cell until it is so full it bursts. So, one virus become thousand and then a gazillion. I know it sounds like a Sci-Fi movie, but it happens.
So there you have a quick field guide to bacteria and viruses.