Every winter, millions of people catch colds, flu, and stomach bugs, and this BBC report sets out to explain why the cold months make us so vulnerable. Professor Alice Roberts and Dr Michael Mosley broadcast from a pop-up studio near London's major hospitals and research institutions, tracking outbreaks around the country with virologists as guides. Norovirus gets the most attention: a vomiting bug that closed hundreds of hospital wards and infected over a million people in the year the program was made, with advice on hand washing and staying home to stop it spreading. The pair also cover seasonal flu, whose case numbers are running higher than the previous year, and RSV, a lesser-known respiratory virus that hits babies and young children hard. Interviews explain what these viruses actually do inside the body and how each one travels from person to person. The advice throughout is practical rather than alarmist: vaccination, hygiene, rest, and knowing which symptoms mean a trip to the doctor rather than a day in bed.