The rapid spread of swine flu has been something of a surprise. Normally influenza is highly seasonal and does not infect large numbers of people until later on in the year. It is typically spread by the coughs and sneezes of those living or travelling together in winter. But summer does see the peak of another type of virus, the enteroviruses, sometimes known as "summer flu". They infect the gastrointestinal system and often produce symptoms similar to influenza including high fever, headaches and weakness lasting for three or four days. They are commonest in summer because they survive longer in sewage. Polio is an enterovirus; other dangerous summer enteroviruses include some responsible for meningitis which starts with flu-like symptoms. Type 3 Parainfluenza, which resembles flu, is another summer visitor.

Certain types of bacterial infection are also commonest in the hotter months. In some cases the seasonal connection is obvious, such as diseases carried by ticks, and food poisoning from the lack of proper refrigeration. With others the reasons are less well understood. One theory is that they are related to the summer lifestyle; e coli thrives in undercooked barbecue food, and swimming in pools and the sea increases exposure to other bacteria. Summer romances may have an effect. Researchers believe that people have sex more often in the summer months, leading to a rise in other types of infection.