Able to mutate, to adapt, to use host cell mechanisms to reproduce themselves with astounding rapidity, unpredictable, viruses sometimes seem to have presented humanity with a veritable challenge. Nearly 100 years after the terrifying Spanish influenza pandemic of 1918-19 that claimed over 25 million victims around the world, research teams still attempt to understand how this virus was able to attain such a huge magnitude. Working on samples of the lungs and brains of victims buried in the frozen earth, they hope to be able to analyze the traces of the fatal virus, preserved thanks to the extremely low temperatures. However this epidemic that left such a mark in history is but one example among many others. The names of the numerous viruses detected around the world over the years possess a threatening resonance: smallpox, poliomyelitis, hepatitis C, dengue fever, AIDS, Ebola, Hanta, Nipah, SARS, Avian Flu. While some, such as smallpox and poliomyelitis are known and have been harnessed, others - AIDS, Ebola, Avian Flu - are the subject of intensive research. New viruses, such as the Nipah virus that recently emerged in Malaysia, and mutant forms related to existing viruses are regularly reported in different parts of the planet. Laboratories in which the world's most efficient scientists work together have launched a veritable race against the clock to learn more about the most immediately threatening viruses and how to control them one day. Important studies are also being carried out by biologists in the field to trace and investigate viruses. Yet, although a virus may be synonymous with disease, it can be positive and beneficial in certain cases, and even an effective agent in the battle against disease, notably cancer. Initial clinical tests with specific viruses suggest that it can halt the growth of cancerous tumors in man.. A few examples illustrating the new paths being explored at present in virology, and that could lead us to reconsider our perceptions of viruses.