A virulent strain of bird flu is gradually spreading around the world, most recently appearing in France.

How far the strain has spread is unknown, but investigators are actively trying to track down the infected man's partners.

Some officials said a more virulent strain of syphilis might be spreading.

The crack-induced strains on American life are spreading.

And if drug-resistant strains spread, ultimately making the medicines ineffective, that would create an even more serious public health problem.

The resistant strain did not spread to anyone else, including a third family member who also had avian flu.

It has been found that the European strain can spread via the import of Christmas trees.

They also told of fresh evidence that the new strain had spread to widely separated points in the prison system.

Now a strain of melarsoprol-resistant sleeping sickness is spreading.

Fortunately, the strain did not spread easily from one person to another.