Watch your frogs for signs of illness. Red leg disease is common. The frog's underside and legs turn red. Contact your vet at any signs of illness. Recognize that frogs do molt, so do not take this as a sign of illness.
Frogs also hibernate.
The frog may appear to be very still and may attempt to conceal itself. Do not disturb your frog when it is hibernating. If it is breathing, it is okay. Talk to your vet or the person you purchase the frog from to get an idea as to what kind of behavior is normal for your frog. If your frog suddenly behaves differently, contact your vet.