Doctors have long been puzzled that certain viral diseases, such as measles, influenza and mononucleosis, and the vaccines against those diseases, can, in rare cases, lead to severe complications such as encephalitis and other diseases of the nerve tissue.

The best known example occurred during 1976, when a national campaign to vaccinate people against swine flu was aborted after a number of people developed a complication called Guillain-Barre syndrome, which afflicts the nerves.

Now a team from the University of Washington medical school in Seattle has found a clue that may explain the phenomenon.

The viruses, they have discovered, contain proteins that are similar in certain ways to some of the proteins that make up the nerves' protective myelin sheath. When the body's immune system manufactures antibodies tailored to attack the virus proteins -- a process stimulated by infection or vaccination with inactivated viruses -- the antibodies may also be tailored to attack the myelin sheath, causing the complications.

The similarities between virus and myelin proteins are far from complete, which may explain why the overwhelming majority of vaccinations and infections do not lead to complications.

The similarities were found by using a computer to compare the structures of known viral proteins with known myelin proteins, looking for small regions of each that match.