发烧 (fāshāo) is a verb meaning 'to have a fever' or 'to run a temperature.' It describes the condition of having an elevated body temperature, usually due to illness. It is one of the most common medical terms used in everyday Chinese.
孩子昨晚突然发烧了,我们赶紧去了医院。
Háizi zuó wǎn tūrán fāshāo le, wǒmen gǎnjǐn qù le yīyuàn.
The child suddenly got a fever last night, so we rushed to the hospital.
你额头好烫,是不是发烧了?
Nǐ étóu hǎo tàng, shì bù shì fāshāo le?
Your forehead is really hot—do you have a fever?
她发烧三天了,还是没有退。
Tā fāshāo sān tiān le, háishì méiyǒu tuì.
She's had a fever for three days and it still hasn't gone down.