It's not - cold and flu (short for influenza) are both contagious respiratory illnesses, however they're triggered by different viruses.
Both viruses attack the cells of the adenoids at the back of your throat, spreading to cells in the rest of the upper respiratory tract (ie, the nose, throat, pharynx and larynx).
"If you have more severe symptoms, a fever of 39.5 and you’re aching all over, it’s more likely that you've got flu," says Sebastian Johnston, professor of respiratory medicine and allergy at Imperial College London. "But flu only really happens during annual epidemics – we normally have one every year [from October to May], they vary in severity and will last six to eight weeks in more highly populated parts of the country before spreading."
Currently, the World Health Organisation (WHO) recommends the flu jab for pregnant women, health workers, people over 65 (or under five) and those with chronic health conditions. But it is available for everyone (ask your GP) and is the only reliable protection.