Shrinking. A liver tumor before treatment with a genetically engineered vaccinia virus (top) and 4 weeks after.Credit: Stephen Thorne/University of Pittsburgh A virus that was instrumental in eradicating smallpox is now showing promise as a potential cancer treatment. A genetically engineered strain of vaccinia, better known as the smallpox vaccine, kept rabbits' liver tumors in check in a new study. The virus is now headed toward trials with human patients. Scientists have been trying to genetically engineer viruses to selectively infect and destroy cancer cells for more than 10 years, but with limited success. The most advanced so far is...