The polio virus, once widely feared for its power to cripple and kill, may be the key to treating deadly brain tumors. A report at the May 2001 meeting of the American Society for Microbiology described how the virus targets and destroys the tumor cells of malignant gliomas, which are among the most common and lethal of brain cancers. An inactivated form of the virus, something similar to a vaccine, has been used to treat brain tumors in animals - all of the tumors shrank away completely within days, with no polio-like side effects. The US National Cancer Institute is working on a polio-based drug for humans, with human testing expected within a few years.