These parasites normally infect the entrocytes of the small intestine and are spread by the orofaecal route.

This happened in 1994 when the canine distemper virus, which normally infects only dogs, suddenly jumped to lions.

The other species of this complex belong to a number of oligotypes and do not normally infect humans.

Types 5 and 6 are most closely aligned with the species that do not normally infect humans.

Avian flu viruses do not normally infect humans.

The researchers study a virus called phi-6, which normally infects a species of bacteria called Pseudomonas phaseolica, a microbe that lives on plants.

If a cell does not express these receptors then the virus cannot normally infect it.

There are four types of the malaria parasite that normally infect people, but only one - Plasmodium falciparum - is potentially deadly.

The bacteria normally infects and grows inside human immune system white cells called granulocytes.

Gynaecotyla adunca is a fluke that normally infects birds.