Plague is one of my favourite killers to go against!

Generally it's best to not cleanse right away (exceptions: death hook, near end game, but make sure it's in areas that would be inconvenient for the Plague to get to) . I had so many solo games where people would just beeline to a fountain, giving her power for free basically. When I'm in a SWF, she becomes way easier to counter.

Or if you see there are 2 fountains left, you can cleanse in one of them, get yourself infected again so that you reset all the fountains (Plague will automatically get her power, but all fountains will be cleansed).

If she is in her power, hide and wait it out. You can also stun her to force her out of her power.