While I don't doubt that a lot of these people might just have underlying unhealthiness, many viral diseases can absolutely knock the wind out of you.

A few years ago, we had a massive chikungunya outbreak in my country. Nearly a dozen people in my extended family caught it, including my father. They all had severe body aches that lasted months, some even years. A professional athlete who had gone through several major sports injuries described it as the worst pain he had ever experienced.