The main Spanish virologist and influenza virus specialist (the influenza viruses), introduces us to H5N1. We explain why it is so easy to spread and how it is able to acquire the necessary features to jump from birds to humans. We will also briefly review recent pandemic caused by a virus experienced flu. And finally we will have the latest findings on H5N1. Research to suspect that this is a virus that will soon join the list of infectious agents. Virus that have appeared in recent decades causing new diseases, and have jeopardized health systems worldwide.