Just to add some SFJoe-ian context here, the H refers to hemagglutanin and the N to neuraminidase, the two proteins found on the outer surface of the influenza virus (neuraminidase is also the target of Tamiflu, the breakthrough antiviral effective at combating flu). Various flu strains are characterized by their H and N isoforms. The H5 isoform of hemagglutanin is rare, so few people have antibodies raised against it, making H5 strains more virulent. Fortunately, H5N8 is relatively mild for humans, but tragic for birds. Because this is the first outbreak since '83, no antibodies exist in the population, hence part of the problem.
Mark Lipton