Let me answer that—I had the same question a while back, too XD There’s a real scientific fact that people often misrepresent: Southeast and East Asia are the global “cradle” of influenza. Due to high population density and close contact with birds and pigs in rural areas, new flu mutations tend to emerge there and then spread to the rest of the world. Since the strains mutate first in Asia, people there get the “fresh,” new virus before anyone else. By the time that same flu reaches other countries months later, scientists have already studied it, and the virus has lost some of its initial contagiousness. That’s where the false impression comes from that colds “hit them harder.” I’ve heard that saying several times, but it’s because of the way it’s portrayed in TV, anime, manga, k-dramas, and everything related to them. The exaggeration of a minor cold into a near-fatal event stems directly from Japanese societal pressures—specifically, the “Obligation to Push Through.” In Japanese school and corporate culture, there is immense pressure to prioritize the collective and not miss obligations. Characters routinely ignore early symptoms, walking around in the rain or overworking themselves until their bodies physically force them to collapse—you know the rest. Ultimately, the dramatic manga fever serves the exact same narrative purpose as Western television’s “stuck in an elevator” cliché—it isolates two characters from the rest of the world so they are forced to deal with their feelings :-P

Are the flu strains more severe in Japan? In every manga, someone gets sick and almost faints from a fever out of nowhere... or maybe it's become a plot cliché there.