Influenza is a contagious respiratory illness caused by influenza viruses. Influenza typically begins with the sudden onset of fever, often accompanied by a cough or sore throat, chills or rigor, headache, malaise, and diffuse myalgia. As the disease progresses, respiratory tract signs, including sore throat, nasal congestion, rhinitis, and cough become more prominent. Less common symptoms such as conjunctival injection, abdominal pain, nausea, vomiting, and diarrhea are more likely to occur in children. Symptoms usually resolve in five to seven days.

Influenza is spread primarily person-to-person through droplets created when a person infected with the virus coughs, sneezes, or talks. Infection occurs when infected droplets land in the mouth or nose of people who are nearby or are inhaled into the lungs. Less often, infection can occur when a person touches a surface contaminated with an influenza virus and then touches their mouth or nose. The influenza virus can survive for hours on solid surfaces, particularly in lower temperatures and lower humidity. The incubation period is usually one to four days, with an average of two days. A healthy adult is infectious beginning one day before and up to five to seven days after symptom onset. Children may be contagious for a longer period of time.

Influenza is a single-stranded, helically shaped, RNA virus of the orthomyxovirus family. There are four types of influenza viruses: A, B, C, and D. Only influenza A and B viruses cause seasonal epidemics and both types are included in influenza vaccines. Influenza C viruses cause a mild respiratory illness and are not included in influenza vaccines. Influenza D viruses primarily affect cattle and are not known to infect or cause illness in people. Humans are the reservoir for influenza B and C viruses. Aquatic birds are the primary reservoir for influenza A viruses. Influenza A viruses can circulate in many different animals, including ducks, chickens, pigs, horses, seals, and others. Influenza A viruses are divided into subtypes based on two surface antigens: hemagglutinin (HA) and neuraminidase (NA). There are 18 different HA subtypes and 11 different NA subtypes. Current subtypes of influenza A viruses found in people are influenza A (H1N1) and influenza A (H3N2). Influenza B viruses are not divided into subtypes, but can be broken down into lineages and strains. Currently circulating influenza B viruses belong to either the B/Yamagata or B/Victoria lineage.

Influenza viruses are constantly evolving. Minor antigenic variation within the same influenza A subtype or influenza B type is called antigenic drift. Antigenic drift occurs continuously and results in new strains of influenza A and B viruses, leading to seasonal epidemics. Antigenic shift involves a major change in one or both of the influenza A surface antigens resulting in a new subtype. Antigenic shifts can lead to a pandemic if the new strain can infect humans and is easily spread from person to person between individuals with little or no preexisting immunity.

Seasonal influenza results in yearly epidemics of varying severity, with sporadic cases or outbreaks occurring outside of typical season patterns. In the northern hemisphere, cases of influenza occur in greater numbers during the winter months, usually peaking from December to March, with a 5% to 20% attack rate in the general population. Some people such as senior citizens, young children, children less than two years of age, and persons with certain health conditions are at high risk for serious complications from influenza. Bacterial pneumonia, ear infections, sinus infections, dehydration, and worsening of chronic medical conditions such as congestive heart failure, asthma, or diabetes may occur. Children may have severe neurologic complications that range from febrile seizures to severe encephalopathy and encephalitis. Reye syndrome has also been associated with influenza infection.

Humans can be infected by influenza viruses circulating in animals. Usually these human infections of zoonotic influenza are acquired through direct contact with infected animals or contaminated environments and do not spread very far among humans. Novel influenza is defined as a human case of I fection with an influenza A virus subtype that is different from currently circulating human influenza H1 and H3 viruses. New subtypes of influenza A can emerge among humans through direct transmission of an animal influenza virus to humans, or through reassortment of genes derived from an animal influenza virus and a human influe za virus. Such genetic reassortment can create a new virus that combines human and animal influenza properties. Human infections with novel influenza A viruses that can be transmitted from person to person may signal the b ginning of an influenza pandemic. The first laboratory suggestion of a novel influenza A infection from an animal source is the inability of available tests to subtype the detected virus.

Influenza pandemics can lead to substantially increased morbidity and mortality rates compared with seasonal influenza. During the 20th century, there were three influenza pandemics, in 1918 (H1N1), 1957 (H2N2), and 1968 (H3N2). The first pandemic of the 21st century occurred from April 2009 to August 2010. The influenza A (H1N1)pdm09 strain has replaced the previously circulating seasonal influenza A (H1N1) strain.

For a complete description of Influenza, refer to the following texts:

• Control of Communicable Diseases Manual (CCDM)
• Red Book, Report of the Committee on Infectious Diseases
• Epidemiology and Prevention of Vaccine-Preventable Diseases, Centers for Disease Control and Prevention