The primary determinant of infection for any virus is its ability to enter a cell and produce additional infectious particles. The presence of CD155 is thought to define the animals and tissues that can be infected by poliovirus. CD155 is found (outside of laboratories) only on the cells of humans, higher primates, and Old World monkeys. Poliovirus is, however, strictly a human pathogen, and does not naturally infect any other species (although chimpanzees and Old World monkeys can be experimentally infected).