Influenza (the flu) is an infectious disease with virus etiology. The symptoms are fever, coughs, headache, myalgia (muscle pain). It spreads easily from one man to another through airborne transmission, it circulates through the whole world and can infect anyone – from kids to the elderly, and can cause epidemic and pandemic (when it spreads globally) outbreaks. There is data that up until now mankind has survived more than 30 pandemics. The most devastating of all happened in 1918 and caused the death of 20 – 50 million people. There are three known influenza viruses – A, B, and C. The viruses that cause substantial epidemics are A and B. The flu reaches its peak frequency in winter, probably due to low temperatures and lower air humidity. The incubation period (the time between getting infected and the appearance of the symptoms) is between 1 and 4 days. The flu has an acute onset, a toxic shock syndrome develops, which sets the flu apart from all other acute respiratory diseases. The complications from the flu may be due to a virus invasion or a secondary bacterial infection. The respiratory, cardiovascular, and nervous systems are impacted. The flu may get complicated by laryngitis, pneumonia, bronchitis, otitis, sinusitis, myocarditis, pericarditis, meningitis, etc. The World Health Organization (WHO) recommends an annual and specific flu prophylaxis through influenza vaccines, best to be administered before the season with increased number of flu cases. There are two types of flu vaccines – inactive (dead) and weakened (live). The live flu vaccine contains attenuated viruses (A and B), which preserve their ability to multiply in the epithelium cells of the upper airways and lead to the acquisition of immunity without causing illness. It is sprayed in the nose 2 or 3 times with a period of 15 days in-between. The immunity lasts between 6 months and up to 1 year. It can be administered in children at the age of 2 years or more. The inactive (dead) vaccine contains a high concentration of epidemic variants of the A and B viruses, killed by formaldehyde and heat. It is applied intracutaneously or subcutaneously. It is suitable for babies of 6+ months. The vaccinated build antibodies. The flu vaccines are produced based on data about currently circulating strains. There are two reasons to get vaccinated annually: flu viruses constantly mutate, also the antibodies built previously by us are gradually getting fewer and fewer – after the sixth month they are cut in half. Adverse reactions to the flu vaccine are rare and mostly local (swelling, redness and pain at the injection site). There are certain groups of children or circumstance where the vaccine should not be administered: those who are allergic to eggs, who have active infections or chronic diseases, and children under 6 months.