Pneumonic plague is a particularly aggressive form of the plague, where you basically need to be treated within the first 24 hours of showing symptoms in order to have much chance of surviving, and you can die in as little as a day and a half. Initial symptoms include fever, weakness, nausea and headaches, aka the same symptoms as probably >90% of the illnesses most of us contract, so I can see how it would be easy to underestimate the severity of it, until you start coughing up blood.

Further, while I don't know this person's circumstances, in a country where there's no guarantee of either universal healthcare or paid sick time and protections for workers who call off work when ill, I can easily see how someone might say "Eh, it's just a cold, but I'll tough it out, because I can't afford to go to the doctor and/or miss work," when they notice some of the milder and less remarkable symptoms, then wake up the next day coughing up blood and already be screwed.