In April 2009 a new influenza virus was discovered, which spread from Mexico to the rest of the world. The new influenza A (H1N1) virus is genetically related to swine flu viruses, and differs substantially from circulating human influenza viruses. It is able to spread from person to person. Because it is a completely new virus, there is probably little immunity in the population. The course of the infection is relatively mild, but the virus will mutate and it is not yet certain whether this will affect severity of the influenza. General practitioners have an important role in surveillance and treatment. The Community Health Services must be notified of any patients who are suspected of having the new influenza. Hygiene measures and administration of antiviral drugs to patients and their contacts may slow the spread. A delay in large-scale spread in the Netherlands allows time for the development of vaccines.