A cytokine storm is the body's reaction to various forms of infections. We might consider it to be an over-reaction, because it is the defensive reaction which kills people rather than the germ itself.
Some people seem to be more prone to it than others. In the case of COVID-19, children with their less-developed immune systems are unlikely to react in this way.
A brief piece on cytokine storms here.
Not related, The Swedish experiment looks like it’s paying off