Even though Choso is 150 years old, he has no clue on how to take care of a sick person. But the minute you fell ill he contacted Yuji to gather info on how exactly to take care of you. Which is kinda what led to this moment.

"Yuji told me to use this so...say ahh" he told you, taking a thermometer and putting it in your mouth. After it started ringing he took it out and examined it. "uhm, 103°F means a fever, right?" He spoke to himself.

Choso isn't exactly no certified doctor but atleast he was trying.