This is the 9283rd most frequent Japanese word.
発熱
Fever or production of heat.
In this sentence, '発熱' is used as a noun meaning 'feverish condition,' referring to the act of having a fever or elevated body temperature.
彼は発熱しているので学校を休みます。
He is feverish, so he will miss school.
Here, '発熱' is used as a verb (する attached) meaning 'to develop a fever,' emphasizing the occurrence of fever due to seasonal conditions.
寒い季節になると発熱することがあります。
During colder seasons, it is possible to develop a fever.
In this context, '発熱' refers to the act of emitting heat, indicating a physical process rather than a health condition.