Viruses are not truly dead or alive. They come assembled in various shapes and sizes. Some cause no effects, and others bring almost certain death. Viruses infect, plants, animals, bacteria, and protists, anything that is alive. They are life's smallest enemies, and although we have noticed their effects for some time, we have only been able to see them for around 30 years.
In the year 1796 the first vaccination was preformed. Dr. Edward Jenner made some small incisions into a boy's arm and then rubbed the non-lethal cowpox virus on the wound. 6 weeks later, after the boy had recovered from his cowpox sore, Jenner infected the same boy with smallpox, which is one of the most lethal viruses we know of. By all reason the boy should have died, but he didn't even get slightly sick. This is because the cowpox virus had made him immune to smallpox. The only thing Jenner had to go on was an old tale that said maids who had gotten cowpox never got smallpox in their life. Luckily for the boy this tale turned out to be true, but Jenner was basically testing his theory. He probably had no idea what would actually happen. Jenner knew little more about how to fight viruses then than we do right now.
Viruses (in a way) helped early American settlers to rid the country of those "pesky" Indians. Within only weeks of the first settlers reaching North America some Indian tribes lost up to 90% of their members due to infectious diseases. They died because most Indian populations had never been exposed (and therefore had no way to fight) foreign viruses like smallpox and measles.
Right now the HIV virus is spreading like a great cloud. It belongs to a type of virus (called a rhinovirus) that can quickly change their internal composition to attack different species. The HIV virus started somewhere in the jungles of Africa, and is now conquering the whole world. Most people who get HIV get the deadly curse of AIDS (which utterly destroys the persons immune system). Some of the lucky few who get HIV don't even get slightly sick or have any symptoms at all (excluding fear).
A virus is neither dead nor alive, which sounds pretty ironic if you think about it. Outside a host (a living cell) a virus cannot move, think, or replicate itself. Outside the host it can't do anything that even resembles life, but when a virus gets inside a host it orders a cell to create new viruses. Sometimes the virus replicates itself so much that the whole cell explodes with infectious particles. The virus acts like a new boss of the cell that has had the old one thrown out.
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A virus consists of at least these two things: a protein coat, and a nucleic acid core (which can consist of either RNA or DNA, never both). Some are built round and small, others have much more complex shapes like the T type bacteriophage which infects only bacteria (my favorite virus).
Most viruses don't seem to do anything harmful. About 1% of our own hereditary material is made up of viruses hitching a free ride in our DNA, but since the viruses that cause diseases are the easiest ones to find and identify they are about the only ones scientists care to categorize and study.
Lately there has been strong evidence linking viruses (in some way) to cancer. Viruses probably do not directly cause cancer, but it has been theorized that without certain combinations of viruses in a body, carcinogens like radiation, and cigarette smoke might have little effect in creating cancer.
The Tobacco Mosaic virus is a very well studied virus that infects tobacco plants. Once scientists discovered bacteria they thought that all diseases might be caused by them, but they could not see the Tobacco Mosaic virus with even the best microscopes of their time. They weren't able to capture the virus in their best filter either (it was simply to small to be filtered). Even though they couldn't see it they were sure it was there because of its effects. It took them many years before they could actually see what they were trying to discover.
Viruses cannot be seen with a light microscope (except for the very largest viruses, which are just barely visible with a very good light microscope), because the wavelength of light is simply too large to distinguish a virus from a medium. It was not until 1930's when the electron microscope was invented (which has a smaller wavelength than visible light) that we were able to see viruses in any detail.Although current medicine has brought us many useful and effective vaccines against viruses, our best defense seems to be our own immune system. When kept strong and healthy, our immune system can fight off almost every virus there is. When our body conquers a virus infection it builds up a natural immunity to it so that it is unable to catch the virus for some time in the future. When our body has a virus (like a cold) we build up mucus in our throats to trap some of the viruses so they can be expelled. Fevers also help kill viruses because they cannot stand being warm for very long.
One of the best ways to stay safe from viruses is to wash your hands and to keep your immune system strong and healthy. You can keep your immune system healthy by eating enough vegetables and minerals (yes, your mom was right), and by taking vitamin C supplements, but one is never truly safe from viruses.
Viruses could theoretically "live" in space and on other planets. Who knows how long they could stay intact, or how far they could travel in the vast vacuum of space. Viruses will probably be the first form of "life" we discover on a place other than earth.
Well there you have it, viruses, one of the smallest, and oldest, things on this planet, and still one of the most misunderstood.