A walk on the wild side
is the title of an interesting article in last week's BMJ about emerging zoonotic diseases. Zoonotic diseases (aka zoonoses) are diseases spread from animals to humans. Malaria would be a classic zoonoses, while some diseases that are now endemic in humans, like measles, probably evolved from zoonotic diseases.
According to the article 75% of emerging infectious diseases are zoonotic, including avian flu, Lyme disease, West Nile and Ebola viruses and some I hadn't ever heard of, like Nipah virus. This is spread to humans by some combination of pigs, dogs and bats (like in the picture). It killed 106 people in Malaysia, but never spread from human to human. A similar virus has emerged in Bangladesh, this one apparently capable of human to human spread.
Why now? One answer is that we are increasingly encroaching into wild lands. I am skeptical about this as humans have always been encroaching on something. I think these kind of disease outbreaks have always occurred, we just notice them more now. Even 100 years ago if 100 people died of an unknown disease, no one really took notice. Now the CDC sends in its hot-shot epidemiologists and we find out what is causing it. More worringly, the rise in air travel has made it a lot more likely that a small outbreak will spread (like SARS) rather than just burn itself out.