krankheitsbedingt
due to illness
Caused or justified by an illness.
Der Mitarbeiter fällt heute krankheitsbedingt aus.
The employee is absent today due to illness.

illness-related
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due to illness
Caused or justified by an illness.
Der Mitarbeiter fällt heute krankheitsbedingt aus.
The employee is absent today due to illness.

The word is made up of 'Krankheit' (illness) and 'bedingt' (conditioned/caused). Here, 'bedingt' means 'caused by'. Literally, it therefore means 'caused by an illness'.
Sein Fehlen war krankheitsbedingt.
His absence was due to illness.
'Krankheitsbedingt' is very often used as an adverb to state the reason for an action. It can also stand as an adjective before a noun, which is especially common in written language.
Sie musste das Treffen krankheitsbedingt absagen.
She had to cancel the meeting due to illness.
This word is often used in professional or formal contexts. Typical combinations are 'krankheitsbedingter Ausfall' (illness-related absence/cancellation), 'krankheitsbedingte Abwesenheit' (illness-related absence), or 'krankheitsbedingt fehlen' (to be absent due to illness).
Bitte entschuldigen Sie meine krankheitsbedingte Abwesenheit in der letzten Woche.
Please excuse my illness-related absence last week.
sick
'krank' and 'krankheitsbedingt' both have to do with illness, but they do not describe the same thing. 'krank' says that a person, or also an animal, is not healthy. 'krankheitsbedingt' explains the reason for something: something happens because of an illness. Learners may confuse the words because both occur in situations involving health. In practice: use 'krank' for a person's condition. For the reason for a cancellation, an absence, or a break, 'krankheitsbedingt' fits better.

Der Termin fällt am Montag krankheitsbedingt aus.
The appointment on Monday is canceled due to illness.
Meine Schwester ist krank und bleibt heute zu Hause.
My sister is sick and is staying home today.