Polio virus.

http://www.livescience.com/52004-polio-virus-gut-shedding.html

PS
A very intelligent question was asked from Dr. Sabin by our batch-mate 'Ooyrilankumaran', during a lecture by Dr. Sabin to medical students in Colombo, in the early 1960s. "Ooyir' while still a medical student in the third year wanted to know from Dr. Sabin, the fate of the live attenuated polio virus vaccine in the epithelium of the small bowel, when we knew that the epithelium was  shed at a rapid rate. Dr. Sabin was taken aback by the question and  wanted to know  whether the  questioner was an MD. On being told that 'Ooyir' was a medical student, Dr. Sabin was more impressed.