Smaller markets are easily swayed by not so much money. Arbitrage is difficult because there is an inherent threat that you're betting against insider information.

In this specific case it's clear that the market doesn't represent our collective knowledge about hantavirus at all. Twenty days ago the odds of a hantavirus pandemic being declared by the WHO were ~10% and now it's 5%. This is just a reflection of the news cycle, it would be nuts to say that this is a reliable estimate for the underlying scientific facts.