We Happy Few

If you're like me and you can't catch plague no matter how many times you get hit or how many bodies you search, here's a guaranteed way to get this achievement:

Playing as Arthur, visit the Apothecary shop in St. George's Holm and talk to the guy at the counter, Stewart Adams. He'll give you a quest called A Sucker for Science (I never had a yellow quest marker for this). Complete this simple quest then come back to him and he'll give you another quest called Mother's Little Helper. Once you've dealt with Mrs. Chaney in this quest, go in the adjoining bathroom and pick up the drugs called Altered Formula Dexipam. Inject yourself with these drugs and tada, you've got the plague (note that doing so will fail the quest, thanks TillTheMorning). Then use a Phenocycline Jab and job done, achievement unlocked.

You'll need a few items to prepare for this achievement. You'll need several healing items and a phenocyline jab. Heads to lud's Holm, the Bobby on guard at the bridge will give you a phenocyline jab. Whilst in lud's Holm, you'll be attacked by a plague infected wastrel straight away, let him keep hitting you until you become infected. It may take one hit, it may take twenty hits, hence the healing items. When you do become infected, use the phenocyline jab immediately and your achievement will pop. If for some reason this guy doesn't infect you, other plague victims only come out at night and have some sort of swarm around thier heads, as well as acting crazy! Also I found wearing the torn suit as opposed to the padded suit got me infected quicker. Hope this helps.

Edit: as pointed out in the comments, you'll know you're infected by a red biohazard symbol appearing in the top left of your husband.

Another Edit: seems a few people keep taking hits without getting infected. Whilst playing as Sally, I couldn't search the body of a plague victim without getting infected, so try repeatedly searching bodies and you could pick up the plague that way!

An alternate method I found was to go to the house a little south west of the Royal Artillery Park in Luds Holm. In there you'll find numerous dead bodies with the plague. If you loot them you'll contract the plague. Pop a Phenocycline jab after and the achievement should pop.

EASIEST SOLUTION.
I found the fastest solution for this.
Fast travel to lods Holm, just outside the fast travel house there are some npc. The ones you need are the people that walk weird and have some stars or something like that above their heads. Kill one and carry him next to the hatch you use to fast travel. Go to sleep for more than 24 hours. Then come outside and loot the body. The body should have mushrooms around its head or on parts of the body. 100% you get the plague. Inject the solution that cures plague and wait for a minute to unlock the achievement.

Had one hell of a time with this one, but actually found a way to get it pretty quickly. What you wana do is when playing as sally, fast travel to rat holes hatch and start knocking enemies unconscious with your “sleeptite atomizer” then search the bodies and pick up everything in their inventory, got the achivment by the 3ed body :) hope this helps y’all out and happy hunting