How did the Thucydides know about symptoms of plague?

There is nothing in surviving sources that suggest Thucydides had any medical training or expertise. He likely observed the symptoms directly, as the epidemic was very widespread and he was in Athens during the outbreak. He may also have spoken with doctors or other medical professionals about the symptoms. Additionally, he could have read about the symptoms in medical texts, as there were a number of medical works in circulation during his time.