Humans can be infected by influenza viruses types A,B, and C. Type A affects humans, birds, and pigs. Type B and C affect only humans. Type C is less severe than type A and it does not cause pandemics.
In this visualization by Information is Beautiful, we can see how the different strains of influenza virus affect humans, what is their origin, and how they are transmitted from pigs and from birds ultimately to humans.
Type A influenza is divided into H and N strains. The “swine flu” N1H1 killed 15,000 people worldwide in 2009-2010. The “bird flu” H5N1 strain, with a fatality rate of 60%, has killed 371 people as of 2013. The more recent H7N9 strain has killed thousand of pigs in China, with 8 human fatalities to date.