Nobody can say with any certainty whether viruses are getting 'worse'. They are, however, extremely adaptable organisms, and it is the way viruses are able to exchange genetic material as well as adapt to different hosts that holds the key to their apparently increasing virulence.
The influenza virus, which is an RNA virus, can mutate up to a million times more often than a DNA virus. Only the human immunodeficiency virus (HIV), supposedly another RNA virus, mutates faster.
In addition to ducks, the influenza virus is also able to infect pigs as well as humans. Strains of duck, human and pig viruses can easily swap genetic material. If a host becomes infected with two or more viruses simultaneously, they can rapidly mutate into new and never-before-seen strains. If one virus is extremely communicable while the other is extremely lethal, the mixing and matching of their genes could produce a lethal hybrid. This is what scientists speculate happened in 1918.
New evidence from Australia suggests that the Spanish flu pandemic of 1918 that killed so many healthy people in their prime was a mutation of human and pig strains of influenza. The researchers' findings are based on a reexamination of gene fragments of the 1918 flu virus, extracted from the preserved tissue of two soldiers who died from it and the lungs of a third victim who was exhumed after 80 years from a grave in the permafrost soil of Alaska (Science, 2001; 293: 1842-5).
As long as we continue to provide environments where viruses from different hosts can easily mix and exchange data (for instance, the Oriental practice of hanging duck cages above pig sties so that the pigs can feed on the avian faeces), viruses will continue to take advantage of this and grow ever more powerful.