The general acceptance of the safety of cross-species produced vaccines was supported in part by the generalization that there are inherent restrictions to the interspecies spread of disease. Thus, like vaccinia, mostviruses are less harmful, but others can be far more dangerous after invading a foreign host. One dramatic example is that of the human infection caused by the herpes-type monkey B virus. This germ remains a rather harmless invader of monkeys, but place it in humans, and striking, severe, acute illness results which commonly ends in death. It is the slow onset of disease that can be particularly baffling, especially when considering potential viral diseases transmitted through vaccines.
Most acute diseases are relatively easy to recognize and amenable to further prevention.
The delayed onset of chronic debilitating diseases that could be associated with animal viruses finding their way into a new species, e.g., man, are much more challenging. Here, the association between the germ and the symptoms it causes is obscured. Such an association would be especially hard to establish if the clinical features presented during the illness are poorly defined and mimic those of other known ailments. One example is the 1996 concern over the food-borne transmission of the prion disease scrapie. Then it was apparently passed on to humans resulting in juvenile Crutzfeldt-Jakob disease.