It is recommended that it be offered to people who have yet to receive a vaccine for the current flu season, who have been vaccinated less than two week since contact, if there is a significant mismatch between vaccine and circulating strains, or during an outbreak in a closed setting regardless of vaccination history. Infection control
Hand hygiene is important in reducing the spread of influenza. This includes frequent hand washing with soap and water, using alcohol-based hand sanitizers, and not touching ones eyes, nose, and mouth with ones hands. Covering ones nose and mouth when coughing or sneezing is important. Other methods to limit influenza transmission include staying home when sick, avoiding contact with others until one day after symptoms end, and disinfecting surfaces likely to be contaminated by the virus, such as doorknobs. Health education through media and posters is often used to remind people of the aforementioned etiquette and hygiene.There is uncertainty about the use of masks since research thus far has not shown a significant reduction in seasonal influenza with mask usage. Likewise, the effectiveness of screening at points of entry into countries is not well researched. Social distancing measures such as school closures, avoiding contact with infected people via isolation or quarantine, and limiting mass gatherings may reduce transmission, but these measures are often expensive, unpopular, and difficult to implement. Consequently, the commonly recommended methods of infection control are respiratory etiquette, hand hygiene, and mask wearing, which are inexpensive and easy to perform.
Influenza, commonly known as "the flu", is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptoms begin from one to four days after exposure to the virus (typically two days) and last for about 2–8 days. Diarrhea and vomiting can occur, particularly in children. Influenza may progress to pneumonia, which can be caused by the virus or by a subsequent bacterial infection. Other complications of infection include acute respiratory distress syndrome, meningitis, encephalitis, and worsening of pre-existing health problems such as asthma and cardiovascular disease. There are four types of influenza virus, termed influenza viruses A, B, C, and D. Aquatic birds are the primary source of Influenza A virus (IAV), which is also widespread in various mammals, including humans and pigs. Influenza B virus (IBV) and Influenza C virus (ICV) primarily infect humans, and Influenza D virus (IDV) is found in cattle and pigs. IAV and IBV circulate in humans and cause seasonal epidemics, and ICV causes a mild infection, primarily in children. IDV can infect humans but is not known to cause illness. In humans, influenza viruses are primarily transmitted through respiratory droplets produced from coughing and sneezing. Transmission through aerosols and intermediate objects and surfaces contaminated by the virus  also occur. Frequent hand washing and covering ones mouth and nose when coughing and sneezing reduce transmission. Annual vaccination can help to provide protection against influenza.