There are four types of influenza: influenzas A, B, C, and D. Influenza A and B are the types responsible for seasonal epidemics or flu season. Influenza C infections are limited to minor upper respiratory infections in children, while D prefers to infect cattle and not humans. Influenza A and B circulate during flu season and infect five to twenty percent of the United States population during these annual epidemics.viii
Influenza A is the predominant type and causes most cases of flu. A wide variety of birds can become infected with it, ensuring worldwide prevalence. Because of the large reservoir in wild birds, influenza A also has ample opportunity for different subtypes to coinfect a single bird and reassort into new subtypes.
Influenza A has two proteins on its surface, hemagglutinin (H) and neuraminidase (N). There are 18 H subtypes (H1 through H18) and 11 different N subtypes (N1 through N11). Each H and N subtype can reassort into new H and N combinations. Reassortment occurs when two different influenza viruses simultaneously infect a single host and share their genetic data. So far, 130 distinct reassortments have been identified, with most occurring in wild birds.ix
Currently circulating human subtypes are H1N1 and H3N2. Subtypes can be broken down further into groups and subgroups (also called clades and subclades) based on their gene sequences. Small changes in these sequences occur over time as the virus replicates until your immune system no longer recognizes it. At this point, the virus has effectively evaded immunity, which is bad for humans but good for the virus from an evolutionary standpoint.
Although influenza A and B both cause epidemics, influenza A is the only known type of flu virus that causes pandemics. An epidemic happens when an infectious disease spreads throughout a community during a particular time. A pandemic is an epidemic that spreads beyond a specific region and into other countries or continents. Pandemics happen when a new virus or bacteria emerges that can spread efficiently and to which humans have little or no immunity.
Some well-known and well-documented influenza pandemics caused by type A influenza are:x
• H1N1: 1918, causing 50 million deaths globally and 675,000 in the US
• H2N2: 1957, causing 1.1 million deaths globally and 116,000 in the US
• H3N2: 1968, causing 1 million deaths globally and 100,000 in the US
• H1N1: 2009, causing 151,700 to 575,400 deaths globally and 12,469 deaths in the USxi
Influenza B is not broken down into subtypes but into lineages and then further divided into groups and subgroups (or clades and subclades). There are two influenza B lineages: Victoria and Yamagata. Type B evolves slower than type A and does not cause pandemics but is still responsible for seasonal epidemics. Influenza B infects just humans and seals,xii which is a much smaller reservoir than the wide variety of birds that carry influenza A. The inability of type B to cause pandemics may be partly due to the limited reservoirs available to the influenza B virus, therefore creating fewer opportunities for mutating into new or novel strains.
How Influenza Spreads
Influenza is a highly infectious virus with a reproductive number, R0 (pronounced R naught), between one and two.xiii This means that for every one person infected, they infect one or two others. R0 is the average number of cases caused per one infected individual and is a way to estimate the contagiousness of an infectious disease.
The flu spreads when droplets from an infected person exit through the mouth or nose and find their way to a healthy individual. If the viral-laden droplets make it into an uninfected person's nose, mouth, or eyes, they may become sick with influenza. People carrying the flu virus can infect others up to six feet away.xiv
Less commonly, flu can spread through fomites (surfaces) such as doorknobs or utensils. If a child with the flu sneezes into their hand and then touches a toy, they may deposit viral particles onto the surface. If a healthy parent later picks up that toy and then touches their nose, mouth, or eyes, they can potentially innoculate themselves with the flu virus and cause an infection.
Once infected with influenza, you are most contagious during the first few days after symptoms commence but can infect others one day before becoming sick and up to seven days after. Asymptomatic infections, or infections that do not present with symptoms, occur in about half of influenza cases and can still spread the disease.xv Asymptomatic and pre-symptomatic transmission should be considered when evaluating influenza prevention strategies.