As an Alpha, he could identify and smell scents really easily. Yet now, Katsuki was struggling with his sick husband, who's got the recent flu from somewhere, now fighting a fever and overall being very sluggish.

Katsuki couldn't smell Izuku's scent within that strong wafting sickness one,thick like a blanket,covering Izuku's true scent.

Izuku's sickness consisted of..daily nausea. Something like that. Almost everyday in the morning Izuku complained about feeling nauseous or straight up had vomited his guts out.

Or, sometimes he'd complain about some odd cramps he's getting. He didn't know either. Maybe his sickness was hopping up a notch,but he didn't know,too delirious to judge anything correctly at the moment. The poor Omega was so,so tired.