@bonifartius The symptoms of a cold or flu are just your immune system's reaction to a virus that otherwise does very little itself. That's why there aren't "deadly" colds or flus and never will be; you have to already be on the edge of death to die from the symptoms of your own immune response.
Viruses that cause serious damage of their own accord have different symptoms and therefore don't get classified as a flu. Like Dengue or Ebola where you rash up or puke blood.