It's fair to say that the world has had more than enough of viruses right now. Unfortunately, the converse is not necessarily true. The incredible vastness of the virosphere is hard to overstate. While several thousand kinds of virus have been studied in detail, scientists say we haven't even scratched the surface. There could be trillions of species overall, some think. Even more conservative estimates are mind-boggling. In the oceans, tens of millions of different kinds may lurk: as many as 10 million viruses can be present in a millilitre of water.