发烧

发烧 (fāshāo) is the everyday verb for 'to have a fever' or 'to run a temperature.' Literally it means 'to emit/develop heat' — 发 ('to give off') + 烧 ('to burn'). Unlike English, Chinese uses a single verb here rather than 'have + noun,' so you say 我发烧了 ('I have a fever now'), not 我有烧 or 我有发烧. The 了 marks the onset of the state.

To give a number, add 度 ('degrees'): 发烧三十九度 ('a fever of 39°C'). The related noun is just 烧 in informal usage (退烧 'bring the fever down,' 退烧药 'fever medicine'). In slang, 发烧 can also describe being passionately obsessed with a hobby — 发烧友 ('enthusiast / fanboy,' as in audio 发烧友 'audiophiles'), but the medical sense is by far the most common.