The natural processes of fecal elimination is handled by the mucosa lining of the colon. When the rectum and colon are traumatized and torn, the person's own fecal matter infects tissue outside the realm of the mucosa lining/rectum. Once infection begins, THEN the virus is drawn to live on unhealthy, infected tissue.
Viruses and bacteria are not the cause; their presence is merely an indicator of infection. For instance, rub fecal matter into a cut on your hand. Will it become infected? Yes, then bacteria are drawn to this to complete the breaking-down processes. Bacteria didn't cause the infection, the infection drew the bacteria.
The area infected and the portal-of-entry of the bacteria/virus dictates which particular strain is identified.