@Charlamandra
What seth means is that every frog have some parasites and worms,
some of them are actually quit good for them,
just as we have good and bad bacterias.
WC frogs ALWAYS are infested with them while CB will have far less.
By putting a WC with a CB you give the oppurtunity to pass on some of the stuff from nature into your "clean"environment.
Another big problem can be between 2 WC species from a different place.
A frog from America may be resistant to types of diseases since evolutionary they geneticaly build up resistance.
Adding for example an Asian frog to it,
you bring in stuff they are not resistant to since it is unknown for their biology,
same goes with the Asian frog that will be prone to catch a typical American disease.
Besides those problems,
any stress will put down the defence mechanism of you're frog,
the parasites and/or worms will boom since the body isn't able to fight it.
Number one cause will be stress, this will weaken any frog significantly.
Adding another frog and even handling them, especially WC, will stress them out and weaken them a lot.
Therefore the parasite or whatever will be able to take control and make you;re frog sick
wich worsens the stress and make it sicker, most people will then get the frog extra attention by picking it up and stressing it even more.
In the end, dead is the result of it.
Best way to deal is,
let the frog be and try to collect feces for examination,
If it soes not poo you can get him/here out of the tank by using gloves
and put it in a little bowl of luke water.
After 5-10 minutes get it out and put the frog in a small enclosure with some kitchen towel or little moist cottonwool.
Most of the time the frog will get something out for you
Then let it examin by microscope as soon as possible to check what is wrong with the frog.
By doing this in time, the vet can often tell you what is wrong and give you a treatment.
Most chance that the frog will make it that way.
Sidenote to that is, i think a frog shoul never be sold sick already.
Before buying it, check for any signs of a disease if possible.
A sick frog can be recognized quit well.
When you got doubts, just don't buy it out of compassion.
A waste of money and the frog will die anyways.