The FDA and the Centers for Disease Control and Prevention (CDC) have launched an investigation into a potential connection between the Gardasil vaccine for the human papillomavirus (HPV) and a rare degenerative muscle disease.

Concern over a connection between Gardasil and the rare disease -- known as amyotrophic lateral sclerosis (ALS) or Lou Gehrig's disease -- was first raised by Phil Tetlock and Barbara Mellers on their blog. Shortly after receiving the Gardasil vaccine two years ago, their daughter Jenny began to lose motor strength and control, eventually becoming completely paralyzed before dying on March 15. Doctors suspect that she suffered from a rare juvenile form of ALS, which affects one out of every two million children.

Government researchers might have taken no further notice, if two other sets of parents had not contacted Tetlock and Mellers with similar cases. In one, a 22-year-old woman died 13 months after receiving the vaccine, apparently from ALS. In the other, a 12-year-old girl who received the vaccine began losing the ability to walk soon after.

"They don't know what she has," her mother said, "but it's destroying her nerves and muscles, and none of the treatments they've given her are working. Before the vaccine, she was a perfectly healthy child, going for her brown belt in karate."

According to ALS expert Barbara Shapiro of the Case Western Reserve University School of Medicine, it is unlikely that the cases are just coincidence.

"Juvenile ALS tends to progress very slowly over years or even decades," she said, "but these girls all seemed to have a more rapid, progressive form."

Shapiro has uncovered what may be a fourth case in the CDC's adverse events database. CDC researchers are now searching the database for other cases, and the FDA has begun to investigate whether a vaccine could trigger ALS.

The CDC has also received reports of ALS developing in people vaccinated against anthrax.