
die Krankheit
illness / disease
A condition in which physical or mental health is impaired; a specific disease or ailment.
Die Grippe ist eine weit verbreitete Krankheit im Winter.
The flu is a widespread illness in winter.
illness / disease
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illness / disease
A condition in which physical or mental health is impaired; a specific disease or ailment.
Die Grippe ist eine weit verbreitete Krankheit im Winter.
The flu is a widespread illness in winter.

illness
The state of being ill as a context for absence or as a reason, often without naming a specific illness.
Er hat sich heute Morgen wegen Krankheit abgemeldet.
He called in sick this morning due to illness.
'Krankheit' (illness/disease) is derived from the adjective 'krank' (sick/ill). The ending '-heit' forms abstract nouns from adjectives that describe a state, as also in 'frei' (free) → 'Freiheit' (freedom).
Nach langer Krankheit fühlt er sich endlich wieder besser.
After a long illness, he finally feels better again.
To describe an illness in more detail, adjectives such as 'schwer' (severe), 'ernst' (serious), 'chronisch' (chronic), 'ansteckend' (contagious), or 'selten' (rare) are often used.
Er leidet an einer seltenen, chronischen Krankheit.
He suffers from a rare, chronic illness.
You 'hat' (have) or 'bekommt' (get) a disease/illness. If you suffer from it, you use the fixed expression 'an einer Krankheit leiden' (to suffer from an illness/disease, with dative).
Viele Menschen im Büro haben die Grippe bekommen.
Many people in the office have caught the flu.
When 'Krankheit' (illness) serves as a general reason for an absence, it is often used with the preposition 'wegen' (because of/due to). In this case, no specific illness is named.
Das Geschäft bleibt heute wegen Krankheit geschlossen.
The shop is closed today due to illness.
to suffer
Leiden and Krankheit can both concern a health-related burden. Krankheit refers to the impaired condition or the specific illness, while leiden expresses that a person experiences pain or hardship because of it. Use Krankheit when talking about the illness itself; use leiden when describing the experience of the affected person.

Die Krankheit nimmt ihr seit Monaten viel Kraft und erschwert ihren Alltag.
The illness has been draining her strength for months and making everyday life more difficult.
Seit Monaten leidet sie an einer Krankheit, die ihr viel Kraft nimmt.
She has been suffering from an illness for months that is draining her strength.