The major changes in the flu virus which cause epidemics occur when a new strain jumps from ducks to pigs and then from pigs to humans. Another human disease transmitted by pigs is the previously unrecognized paramyxovirus, a Hendra like virus, in Malaysia. 3 Many animals harbour diseases which can infect humans, and as is shown in the case of flu, these animals can play significant roles in the creation or mutation of these diseases. Furthermore, science is only beginning to understand all of the possible classes of disease causing agents. Mad Cow Disease may be caused by prions, a type of pathogen quite different from bacteria and viruses.
Prions are extremely resistant to both heat and chemicals.
Additionally, they may or may not be transmitted through blood. Just as HIV went undetected then, prions may be going undetected now, along with any other disease causing agents that have yet to be identified. Daniszewski , Hank. UWO pioneers animal transplant. London Free Press.