Influenza is a respiratory infection caused by the influenza virus. The symptoms are similar to a cold but much more severe. For influenza, the symptom onset is severe, while for a cold it is gradual. Influenza symptoms include fever, chills, aching joints, and sometimes a running nose and sore throat. A cold might include slight aches and weakness but always includes sneezing, stuffy nose, and sore throat. The influenza virus is actually a diverse collection of closely related single-stranded RNA viruses that infect animals. Virus particles are transmitted between people through aerosols that are spread through sneezing and coughing. They are also transmitted through contact with contaminated bodily fluids, such as when a sick person wipes their nose with their hands and touches objects, leaving virus particles behind. There are three major groups of influenza: A, B, and C. Influenza A and B cause the seasonal influenza disease. New strains of influenza A virus infect people and can cause an influenza pandemic. Influenza C is mild and has not been known to cause epidemics. The virus infects epithelial cells in the nose and throat, though more virulent strains of the virus are able to infect a wider variety of cell types. Infection stimulates an innate and adaptive immune response, resulting in the clearing of infection after one to two weeks. The body also produces antibodies specific to protein structures that exist on the outside of the virus particle, such as hemagglutinin and neuraminidase. Antiviral medicine can be used to reduce symptoms of influenza.
However, influenza outbreaks occur every year, and humans are not immune to subsequent infections. This is because the viral genome and structure changes due to mutation which enable some strains to evade the adaptive immune response. Through a process called antigenic drift, mutations accumulate in a viral genome and lead to antibody avoidance. RNA-dependent RNA polymerase, the enzyme that copies the virus genome during replication, has evolved to introduce nucleotide substitutions into the genome at a much higher rate than other enzymes that replicate strands of nucleic acids. When virus genes are transcribed, these mutations cause substitutions in the amino acid sequences of the proteins, changing their structure and allowing the virus to avoid recognition by antibodies specific to previous iterations of the same virus species. Additionally, during infection entire portions of the virus genome from different virus strains can accidentally swap between each other during assembly, leading to a mismatched virus particle that can evade adaptive immunity.
Influenza is named based on two proteins on the virus: hemagglutinin and neuraminidase. There are 14 versions of H protein and 9 versions of N, which means there are 144 varieties of the flu. Flu strains are named after their H and N protein. For example, H1N1 is the name of a strain of influenza that is also known as swine flu because it originated in pigs. Bird flus are strains of influenza that infect birds and humans.An aggressive strain of influenza can cause an epidemic when it spreads rapidly to infect greater numbers of people than is typical or a pandemic when the epidemic spreads worldwide. The Spanish influenza pandemic occurred in 1918 from the H1N1 strain and resulted in the deaths of 50 to 100 million people. The lack of proper sanitary care is one hypothesis for why it was such a deadly pandemic. The Asian influenza epidemic of 1957 started in East Asia but eventually spread across the world. The Asian influenza was caused by the H2N2 strain and caused 1 to 2 million deaths. The Hong Kong influenza epidemic of 1968 originated in China but spread rapidly and caused 1 to 4 million deaths. It was caused by influenza A subtype H3N2. In 2009 a swine flu, H1N1, pandemic started in Mexico and caused as many as 0.5 million deaths worldwide. In contrast to typical influenza epidemics, most deaths resulting from the 2009 H1N1 pandemic were of individuals under age 65.