Causes
Although many people confuse flu and colds or use the words interchangeably, both are caused by specific viruses. A variety of viruses cause colds, whereas flu is primarily caused by the different types of influenza viruses.
There are four distinct types of influenza viruses, not all of which cause disease in humans. These include;
Influenza A: This virus causes the majority of cases of influenza at each flu season. The severity of the flu season also depends on which strain of influenza virus A is circulating. The two strains currently affecting humans are influenza A (H1N1) and influenza A (H3N2).
Influenza B: These viruses may also lead to regional outbreaks during the flu season, but mostly to a lesser degree than A strains. Infections secondary to influenza B viruses appear to be less serious than A strains, but they can still be harmful, particularly for young children and in those with a weak immune system. The strains of influenza B circulating among humans derive from two lines: B/Victoria and B/Yamagata.
Influenza C: Influenza C viruses can affect humans, but they are usually mild and rare.
Influenza D: This strain doesn’t cause infection in humans and is believed to affect livestock, in particular cattle.
Flu viruses also change their structure frequently, making it difficult for the human body to combat them effectively. This is the reason why flu vaccines are updated periodically to include new strains of influenza A and B viruses that are most likely to cause epidemics.