Can someone please explain nicely the best way to play against the Plag?

Typically if it is a map with a lot of low walled loops ie. Wreckers we just stay injured. High walls or indoors cleansing is a pretty good idea.

If playing solo I just stay injured until someone else cleanses.

This, this, this. There is a time and place for cleansing and not cleansing. There's no 100% foolproof Plague strategy. If nobody on the team can loop, you're going to get wrecked by staying injured and you're better off taking your chances with cleansing and immersion. There are certain maps that are difficult to hit Corrupt Purge on. Rule of thumb: if it's a bad Nurse map, cleanse. Hawkins, Lerys, Haddonfield, MAYBE the Game...cleanse.

Keep an eye on how many pool auras you can see. If the next cleanse is the last pool, do it no matter how sick you are. It will force her to become corrupt straight away, and reset all pools which basically loses her 5 uses of corrupt. If you're lucky others will realise this has happened and you basically have a reset on whether anyone is going to cleanse or not.

I typically stay sick against Plag until at least two gens are done. At that point, I will try to cleanse in a pool that is near a completed gen, or in a corner of the map. This works best, in my experience, because she has to go to what is essentially a dead zone of the map to gain her power. She'll waste about half of her Corrupt time simply heading back from the fountain and finding a survivor, and if that survivor is halfway competant, she'll waste her entire power with one with, maybe no hits. There are some maps, like the Swamp maps, where it seems like every fountain is in an obscure corner with no gens near, and those maps are where you can render her power almost useless.

My personal playstyle when I know I have a good team.

- Don't cleanse when there is no reason to.

- If someone is on a hook, one should cleanse on a pool as far away as possible from that hook.

- When you hear the "drrrrinnng" sound, play super stealthy for the next minute, Plague can be seen easily because of the sparks which get out of her head. After that, she wasted her corrupt Purge and maybe everyone can cleanse at certain times as long as there is stealth involved while she has Corrupt Purge

General rules to follow vs a Plague:

• Cleanse if you're on death hook or all generators are powered and you need to escape.
• If you do cleanse, do it as far away as possible from objectives, this forces Plague to leave everything important just for a power-up.
• If you cleanse, then don't do something that immediately gets you sick again, as it ruins the point of cleansing. I see this a lot where people will cleanse and then do a hook save on someone who's sick, making the cleanse pointless.
• At the start of the match, avoid getting sick at all costs as Plague getting everyone sick is her number one priority. This means avoid touching infected objects and also getting hit by her vomit, which leads into my next point.
• In chases at the start of the match before you're sick, treat Plague like Huntress, and break LOS on her in a chase constantly, take corners very sharp, use walls, crouch behind objects when necessary, and be aware of what's infected like windows and pallets so you don't get sick by accident. The longer she can't infect you, the more time she wastes.
• Once sick, don't try to be stealthy vs Plague, you're incredibly loud even with Iron Will and she can find you easily through the coughing and also green cloud of sickness around you.
• If she has corrupt purge, then treat her like Huntress again in chases.
• Not many are aware of this, but pallet stunning a Plague while she has corrupt purge will cause her to lose corrupt purge completely. A good Plague won't ever fall for this, but it's still helpful to know.
• If Plague does take corrupt purge and there's no easy escape like an exit gate or hatch, then treat it like a tier 3 Myers and go into areas Plague shouldn't be patrolling, like an already completed gen, this way she has to commit away from her objective just to try and get an easy down on you.
• If other teammates cleanse when there's no reason to, then you should cleanse, and try to get sick again, then cleanse again so that you can hopefully reset all of her pools and give her just one power-up for 60 seconds. This will take more than one teammate to do, though.
• Be aware that many loops in this game Plague can't mindgame because she's way too tall, so just tilt your camera up a little and see if you can see her.
• If you feel like corrupt purge is about to end, but you're not sure, then look at her censer, if there's still red particles coming from it then that means she still has corrupt purge.

I think people rule out cleansing too much. The plague is a double edged sword of a killer, but cleansing is much better against a plague then people give it credit for. You could be one shot all game, make tons of noise, and do everything while sick to give no chance of hiding or escaping a Chase (unless it's haddonfield or the stranger things map with the infinites), or you could play like normal, but go up against a killer with what is essentially a shotgun as a weapon with the way she can just spread her corrupt purge around however she pleases.