Associated with the most terrible diseases and with the epidemics threatening the whole humanity, viruses have bad publicity. But are they only evil? The answer is no. These micro- organisms, which have remained unknown for a very long time, have gradually revealed their surprising capacities to help the prosperity of our species. About half of our genetic inheritance is composed of sequences of viral origins. More surprisingly, a virus that we all hold in our genetic inheritance plays an essential role in the formation of the placenta. Also, some viruses can be invaluable tools to treat genetic diseases or cancers. The world of viruses is eventually revealing its mysteries.