Oh I wondered about Mystique being strong too then I thought anout the nature of the horsemen and their original names...see where I'm going???
War- Mystique could easily cause this changing into world leaders

Famine- Storm need I say more?

Death- Magneto can we say Magneto all that he's been through and he has no problems killing people himself

Pestilence-Xavier can easily cause mental illness and weaken the immune system as well if he chose

I wanted to add this before but I was pressed for time...hope that helps:)

Void