1.   In some rare instances, however, children acquired polio from the weakened virus.

2.   Infants who received it developed strong protection against polio, and they often passed the weakened virus on to others.

3.   It involved an infusion of trillions of particles of the OTC gene, tucked inside a weakened cold virus.

4.   People infected with weakened AIDS virus in Australia begin to show damage.

5.   A genetically weakened virus that many hoped would be a safe and effective vaccine against AIDS instead appears to mutate into a strain that actually causes the disease.

6.   An outbreak of polio in Haiti and the Dominican Republic has been traced to a weakened virus in a vaccine that mutated and spread disease among poorly immunized children.

7.   Doctors believe the spray works better because it is made with a weakened virus rather than a dead one.

8.   However, the new work shows that over time, the weakened virus can mutate into a lethal form that causes the disease it was meant to prevent.

9.   In the latest research, Nabel and colleagues boosted the DNA vaccine with a weakened virus that normally causes respiratory infections.