Man flu is a colloquial term referring to the perception that men experience more severe symptoms when afflicted with the common cold or flu-like illnesses. Primarily British, it is frequently used in a self-deprecating manner. This term has sparked both public interest and scientific inquiry into potential sex differences in the experience of respiratory illnesses.

The term 'man flu' is believed to have originated in the 1990s, with the earliest known use found in a message posted on the Usenet newsgroup misc.health.diabetes in 1999, as documented by the Oxford English Dictionary. Since then, the term gained recognition in popular culture and became the subject of discussions surrounding the perceived severity of symptoms experienced by men during respiratory illnesses.

An open-access poll featured in Nuts magazine in 2006 brought attention to this notion, although it was widely criticized as unscientific. The poll reported that men reported longer recovery times from flu-like illnesses compared to women, with men taking an average of three days to recover, while women recovered in 1.5 days. A poll conducted for a painkiller manufacturer in 2008 suggested that such exaggeration is just as prevalent in women.

The concept of 'man flu' has sparked controversy, notably in a Boots advertisement featuring the tagline "when he's ill and you don't have time to be – get cold and flu products for just 99p at Boots." This ad ignited debate about gender stereotypes and perceptions of illness in popular culture.

While initially dismissed as a cultural trope, the notion of 'man flu' has prompted scientific investigation into potential physiological differences between men and women in their response to viral infections.

One study, published in 2009, investigated the effects of the bacterium Listeria monocytogenes on genetically modified mice. While the study did not pertain to cold or flu viruses, but rather to a bacterial disease, it gained widespread attention and was misinterpreted by various media outlets, including The Daily Telegraph, as supporting the existence of 'man flu' in humans.

Subsequent research has yielded more nuanced insights into the interplay between sex, hormones, and immune responses. According to researchers at the University of Cambridge, evolutionary factors may have led women to develop more rigorous immune systems than men due to differing reproductive strategies. In addition, a 2011 study conducted at the University of Queensland suggests that female hormones, such as oestrogens, aid pre-menopausal women in fighting infections, but the protection is lost after menopause.

The scientific community continues to research the validity and significance of perceived sex differences in symptom severity and immune response to viral illnesses.