Illness, disease, or sickness; often refers to conditions that may require care or treatment (not usually trivial ailments).
彼は重い病気で入院している。
He is hospitalized with a serious illness.
インフルエンザは冬に流行する病気だ。
Influenza is a disease that spreads in winter.
早めに治療すれば、その病気は治る可能性が高い。
If treated early, the disease is likely to be curable.
家族に遺伝性の病気があるかどうかを確認した。
I checked whether there are any hereditary diseases in the family.
Used figuratively to describe a bad habit, behavioural weakness, or failing (e.g., 'a vice is like a disease').
借金癖は一種の病気だと考える専門家もいる。
Some experts consider a habit of getting into debt to be a kind of illness.
彼の潔癖症はほとんど病気のようだ。
His obsessive cleanliness is almost like an illness.
Colloquially used to mean 'crazy' or 'insane'; a casual/derogatory use that can be offensive.
そんなことをするなんて、病気じゃないの？
Are you crazy to do something like that?
あの人、最近変だよ。病気なのかな。
That person has been strange lately. I wonder if they're sick (or 'off').