Pathogens are usually a normal part of biodiversity, playing their normal ecological role in animal and plant populations. Weak individuals will be eliminated, the strong ones recover or are not even showing signs of infection. That is the outcome of co-evolution, the pathogens develop, the hosts develop countermeasures, the pathogens develops countermeasures against the host countermeasures etc., an arms race, but natural. Human mobility has broken that evolutionary arms race by moving around pathogens (often unknowingly) around the world. If such a pathogen then infects a naive individual, the effects of such an infection can be very strong, in the worst case leading to the death of that individual (how ever strong it would have been in its usual environment). That is what we see on a global scale with the amphibian chytrids Batrachochytrium dendrobatitis and Batrachochytrium salamandrivorans. They cause mass mortalities in amphibian populations. We know of such in two of the P³ focal mountain ranges: Pyrenees and Sierra Nevada, but also from amphibian hotspots such as Madagascar and French Guiana.
Both chytrids originate in Asia, and for Batrachochytrium dendrobatitis we could now pinpoint the origin to Korea. The Science paper makes one point very clear: we know only the tip of the genetic variability of these pathogens. More virulent ones may still exist or may come to exist by hybridizing with other lineages usually geographically separated. Trade brings those together, with unknown and unpredictable outcomes. Banning trade of amphibians is a drastic but necessary step.
A data set like that used for the Science paper is very difficult to retrieve, as it takes many samples to successfully cultivate Bd. The latter is mainly due to the fact that Bd outside of its host is a very weak organism and can be easily outgrown by many other microorganisms. Especially in tropical areas it is therefore tedious work to sample and isolate Bd. The method for isolating Bd is now described and published in Scientific Reports under the lead of Matthew C. Fisher.
From the Science paper:
Species in the fungal genus Batrachochytrium are responsible for severe declines in the populations of amphibians globally. The sources of these pathogens have been uncertain. O'Hanlon et al. used genomics on a panel of more than 200 isolates to trace the source of the frog pathogen B. dendrobatidis to a hyperdiverse hotspot in the Korean peninsula (see the Perspective by Lips). Over the past century, the trade in amphibian species has accelerated, and now all lineages of B. dendrobatidis occur in traded amphibians; the fungus has become ubiquitous and is diversifying rapidly.