1.   No one knows how humans even become infected with the animal virus.

2.   A major unknown is the smallest number of spores it takes to infect a human.

3.   A prevailing belief is that HIV was around for a long time and that on many occasions humans became infected when they killed monkeys or other primates for food.

4.   Adult ticks can also infect humans, but less readily.

5.   But can cow prions infect humans?

6.   But there is mounting evidence that the virus infects humans and is associated with certain types of human tumors.

7.   Flu viruses originate in waterfowl and commonly move through several species, chiefly pigs, before infecting humans.

8.   Henney said FDA researchers are looking at whether animal organs might carry viruses that could infect humans.

9.   His facility is designed to handle investigations of zoonoses, the diseases of animals that also infect humans.

10.   Humans are infected as bystanders.