When the causative virus weakened so that it dropped be- low the threshold of effectiveness, the body's natural defenses were able to repel it.

You can't get the disease from the vaccine because the viruses or bacteria are dead or severely weakened.

"The virus traveled to her heart muscle and weakened the heart muscle," Ms. Loeb said.

The virus weakens a person's ability to fight infections and cancer.

We say someone has AIDS when the virus has weakened the body so much that certain new illnesses are developing.

If so, will the virus weaken and die out?

The morbilli virus, which causes measles in humans, weakens the dolphins and impairs their sonar navigation systems.

The virus also weakened the bulb, so that it produced fewer offsets, or bulblets, for new tulips.

A mild illness similar to a cold also can be caused by the live, weakened viruses in the influenza nasal spray vaccine.

Some people may develop AIDS within a few months as the virus quickly weakens their immune system.