The flu is actually caused by several kinds of viruses that are constantly undergoing genetic mutation and evolving.

The two main kinds of flu virus are types A and B, which can be further categorized according to the proteins found on the surfaces of these viruses. Most people do not die from the flu. On average, people who are otherwise healthy and get the flu can exhibit a range of symptoms, including headaches, body aches, fevers, fatigue, nausea, coughs, congestion, and runny or stuffy noses. However, there are some people who are more at risk for developing worse symptoms, complications, or even possibly dying. Examples include infants, elderly people, or pregnant women, whose immune systems are not well-developed, or compromised. I hope this helps!

A healthy adult human will survive a common flu, but there have been especially deadly flu strains that have killed up to a few percent of a nation's people (the 1918 flu epidemic killed more people than World War I). Even common flu can dangerous to people who are old, sick for other reasons, or have weak immune systems.