Our Jack Russel/Chihuahua mix might be sick..?

I want to know what are the most common ailments and sicknesses in both breeds: Jack Russel and Chihuahua's. Because we have a Jack Russel Chihuahua mix. He is starting to cough more often and it sounds like he has a bunch of 'junk' in his throat, I can hear it when he breathes in and out, Like one of us having bronchitis. Again I want to know the common sicknesses and ailments in both breeds please.