Unfortunatly I have had my share when it comes to diseases. I quarantine every new animal and allways get there faeces tested. Only when I have had 2 negative results do I consider them as 'clean.'
My qt room is a different room. I adjust all care practices in such a way that the animals in qt are the last in line. I dont touch anything in the qt terrariums unless it is absolutely necessary. If I do have to touch anything, I wash my hand before treating the next qt tank with a desinfectans. They all have there own water spray bottle. The tanks are set up in line with there 'suspiciousness' or according to the severeness of there infection. After treating all my qt tanks, I wash my hand again with a desinfectans (usually dettol).
I buy my live food items at a store that does not have any live animals.
I am probably a bit to extreme in this but I have really had some bad luck. About 50% Of all new animals that I buy seem to carry parasites (!). They are found mainly in my new frogs (3 out of 4 new projects), a bit less often (40-50% of all new projects) in faeces of geckos that live in moist or semi moist climate (like chahoua) and only sporadically in faeces of geckos from drier climates. At least that seems to be my experience.
Now I do believe that animals can live a long and happy life as carrier of some parasites, but they need considerably more food to stay in prime form, they go down hill fast if something happens (extreme stress, old age, eggbound etc) and tend to lay less clutches or calcium crash more easily (again this is my experience, other people might have other experiences).
I also believe that animals can carry parasites unnnoted in very very low quantities. Only after a stress periode do they seem to have detectable levels of parasites in there faeces. It is the reason why I test all new animals twice, the first time not earlier then 10 days after purchase.
Ps: i only treat animals when they are subadult. I have found that young animals don't allways grow the way they should if they have been treated. So sometimes I rear up infected young animals in qt untill they are at least subadults. That gives me the best results.