Ill and Healthy
Болен is a SHORT adjective, like рад and занят — predicate only, never before a noun. It agrees: болен (m), больна (f), больны (pl).
Он *болен.* — He is ill.
Здоровый is its opposite and an ordinary long adjective, so it behaves normally: здоровый человек.
| A healthy person is not ill. | Здоровый человек не болен. |
Health Itself
Здоровье is the noun — health. It is neuter, and Russians use it constantly, not least in toasts.
| Sport is health. | Спорт — это здоровье. |
At the Doctor
Температура means both temperature in general and, on its own, a FEVER:
У меня температура. — I have a temperature.
Russian says this with у + genitive, the possession frame you already know — literally "at me a temperature", with no verb at all.
| I have a temperature. | У меня температура. |
Живот
Живот is the stomach or belly. It is worth knowing early for the doctor's surgery, and it is a false friend for anyone who knows another Slavic language, where the same root means life.
| This is the stomach, and this is the face. | Это живот, а это лицо. |
