erkrankt
ill
Describing the state of having an illness. It is often used with 'an' to name the specific illness.
Er ist an einer schweren Grippe erkrankt.
He is ill with a severe flu.

ill
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ill
Describing the state of having an illness. It is often used with 'an' to name the specific illness.
Er ist an einer schweren Grippe erkrankt.
He is ill with a severe flu.

The word 'erkrankt' (ill) is the past participle form of the verb 'erkranken' (to fall ill). The prefix 'er-' here describes the beginning of a state, meaning the transition from a healthy to an ill state.
Leider ist mein Kollege plötzlich erkrankt.
Unfortunately, my colleague suddenly fell ill.
To name the specific illness, use the preposition 'an' followed by the dative. This structure is very fixed and important for correct usage.
Viele Menschen sind an der Grippe erkrankt.
Many people have fallen ill with the flu.
'Erkrankt' (ill) is usually used as a predicative adjective with the verb 'sein' (to be). The construction 'jemand ist erkrankt' (someone is ill) describes the state of being sick.
Der Firmenchef ist seit letzter Woche erkrankt.
The company director has been ill since last week.
sick
'krank' and 'erkrankt' both describe that someone has an illness. 'krank' is the normal everyday word and fits in almost any simple situation. 'erkrankt' sounds more formal and more precise, especially when you name the illness with 'an'. For example, you say 'an Grippe erkrankt' (ill with the flu). Learners may confuse the words because both refer to the same state. In conversation, people usually say 'krank'; in news reports, doctors' letters, or official texts, 'erkrankt' sounds more natural.

Der Patient ist an einer schweren Grippe erkrankt.
The patient is ill with a severe flu.
Meine Kollegin ist heute krank und kommt nicht ins Büro.
My colleague is sick today and isn't coming to the office.