This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/), which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. za, which in most cases is mild, which can be effectively treated with antiviral drugs and for which a vaccine is now available. By the end of 2009 many countries were still reporting disease activity and an impact on health-care services [2] .