The phrase "nasty flu" refers to a very bad or severe case of influenza, a contagious illness that affects the respiratory system and makes people feel very sick. Full definition
Executives at several seniors housing REITs say the burdens of more supply coming on - line this year are being complicated by a particularly nasty flu season. (nreionline.com)
Wish I could spend more time outside but I'm stuck inside because of a very nasty flu. (dcinstyle.com)