Psoriasis - even the fairly innocuous, although annoying, type I have all over my scalp - is a symptom of the immune system attacking healthy tissue in the body.
It's a disorder of immunity. Don't fixate on the external symptoms - the threat is in the fact that it is caused by a disorder in the immune system.
People can get psoriatic arthritis, which is nasty and debilitating (google it), which is why some patients take strong medications that suppress the immune system to stop it attacking healthy cells.
It is not 'a bit of flaky skin'