The disease that afflicted the Beast Tamers' was indeed not contagious but something caused by water contamination.
When they followed all the Beast Tamers' drinking water sources, they discovered that the disease was caused by plants that grew at one of the springs which attracted newly migrated animals from other mountains.
Those animals' main food was the said plant so they visited the spring often to eat, and later, released their urine and fecal matter there, which caused the disease.
As such, the plants were cleared and transferred elsewhere away from the springs so that the disease won't keep recurring.
Even after it was cleaned, the said spring was prohibited from being used for a while to ensure that the disease won't happen again.
The Beast Tamers were fine with it since there were a lot of water sources in the Grada Mountains.
