Scientists long anticipated that an influenza virus will spread - as now appears to be happening in Asia - from wild birds to humans, causing a pandemic. But few predicted last year's SARS epidemic, a pneumonia caused by a coronavirus . Prior to 2003, coronaviruses ranked very low on the scale of important human diseases, mostly being associated with the common cold. But we have long known that coronaviruses can cause dreadful diseases in domestic animals. We simply haven't learned our lesson.
Indeed, events of the last couple of decades - the AIDS viruses transferring from monkeys to man, followed by their global spread, being just one example - should have convinced us that, where diseases are concerned, the unexpected will happen. After all, the "jumping" of viruses from wild animals to humans is more common than we like to think.
All types of virus strains or variants exist - somewhat akin to dogs existing as different breeds. Veterinarians and farmers have long known of a coronavirus that can kill 90% or more of young pigs. Less well known is the fact that cats and dogs are infected by a coronavirus that can also cause disease in pigs.