Smallpox, aids, spanish flu, ... these diseases that fear us came from virus. We would rather livein a world without any virus.... and we would live very bad in this world ! Some kinds of virus are dangerous, sometimes deadly. But in the multiplicity of virus, others don't do us anything, or even make us some good ! In fact, we know for the moment 6,000 virus (but some think that it only represents 1% of the whole), whose « only » 200 are noxious.
First, whats is this ? A virus is an biological entity mainly composed of an envelop (made with proteins) and a few pieces of DNA. A debatte happens, to know if virus should be considered as alive or shouldn't : they own DNA under the same form that our (they haven't any chromosome, but the DNA is chimically the same) but they don't reproduct. More precisely, the can only reproduct inside an other organism, which has to be alive, and that we call host. So, virus come in the cells of their host, they reproduce in it (using the proteinal ressources of the host) and, in most of the cases, thay leave. So, the reason why some of them are dangerous is that they stay in a cell that can't assume its mission in our organism anymore.
But all are not awkward ! Some of them come to multiply and leave our corps without any influence on our health. For example, there are virus that destroy our cells, but only the cancerous ones ! So they possibly can help us to fight against tumors.
Virus can also help us less directly. They are precious for medical and genetic research, because by modifying their DNA, we can build a lot of molecules to work on. And about exobiology (research of an extraterrestrial life), a way to search for life is to search for virus : if there are some on a planet, there shouls be alive organisms.
Some researchers even think that they are essential for the development of life. They could be responsible of the genetic diversity of the populations, because they can transport DNA. They also could be the origin of the life under its current form : our cells contain DNA that permit us to synthetise essential proteins. But the DNA is prisonner of the cores of our cells (it's better for us that the DNA is always at the same place !), and the proteins are synthetised at the outside of the cores. So, the communication is made by RNA, a molecule that can enter and leave the core to transmit the orders of DNA. We currently admit that at the origins, organisms didn't have DNA, but only RNA. There would later had happened an improvement with the creation of the DNA, more resistant than RNA to the changes. And, whereas all alive organisms have DNA, there are virus with DNA and virus with RNA ! So we ask : could DNA be a creation of virus ?

(adapted from Science et Vie number 1078, July 2007)