der Durchfall
diarrhea
An illness in which you have very frequent and watery bowel movements.
Nach dem Essen im fremden Land bekam er leider Durchfall.
After eating in the foreign country, he unfortunately got diarrhea.

diarrhea / flop
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diarrhea
An illness in which you have very frequent and watery bowel movements.
Nach dem Essen im fremden Land bekam er leider Durchfall.
After eating in the foreign country, he unfortunately got diarrhea.

The word is made up of 'durch' (through) and 'Fall' (fall). The literal idea of 'Durchfallen' (falling through/failing) fits both meanings: the illness and failing an exam or project.
Sein erster Versuch, die Prüfung zu bestehen, war ein kompletter Durchfall.
His first attempt to pass the exam was a complete failure.
In a medical context, the fixed phrase 'Durchfall haben' (to have diarrhea) is almost always used. An alternative, but more formal, is 'an Durchfall leiden' (to suffer from diarrhea).
Ich kann heute nicht zur Arbeit kommen, ich habe schlimmen Durchfall.
I can't come to work today; I have severe diarrhea.
The meaning 'Misserfolg' (failure) is colloquial. In formal or written contexts, you would use other words to describe a failure.
Die neue Marketingkampagne war leider ein totaler Durchfall.
Unfortunately, the new marketing campaign was a total flop.
bankruptcy / flop
'die Pleite' and 'der Durchfall' can both colloquially mean a major failure. 'Pleite' sounds very natural when something turns out badly or is disappointing. In addition, 'Pleite' can also mean a financial collapse. 'Durchfall' usually means an illness. Only in colloquial language can it also mean that something was a complete failure. Learners may confuse the words because both can stand for a flop. In everyday life, however, it is important to remember: in health contexts, 'Durchfall' means the illness; for a failed plan, 'Pleite' or 'ein Reinfall' (a flop) usually sounds more natural.

Nach dem Essen im Urlaub hatte Tom zwei Tage lang Durchfall.
After eating on vacation, Tom had diarrhea for two days.
Die Party war eine echte Pleite, weil fast niemand gekommen ist.
The party was a real flop because hardly anyone came.