The illness we commonly know as flu is caused by the influenza virus. There are different strains of the virus, and each year in autumn and winter, one or more new types of virus circulate, spreading among communities and infecting lots of people. This is known as seasonal flu: if you catch a flu-like illness during winter it's likely to be this new, seasonal flu virus.
This is a companion discussion topic for the original article at https://patient.info/news-and-features/when-to-seek-medical-treatment-for-flu