To get this achievement, you need to have Disease, Fatigue, and Poison active at the same time. It's currently one of the three least unlocked achievements in the game, along with the "Lord of Murder" and "Flying Solo" achievements.
An easy way to get this achievement is to start a new game in Throne of Bhaal. Any difficulty is fine. Create your character with as low a Constitution score as possible. Play through the intro so that you get to the hellish pocket plane. At this point, make sure to disable Story Mode in the Gameplay settings if it's on - Story Mode makes the characters immune to most status effects.
Next, summon the mage Edwin and the ranger Valygar. Have Edwin prepare Haste and Contagion for all his level 3 and 4 slots, then sleep. After waking up with the newly prepared spells, cast Haste on your main character. Once the Haste effect expires, the character should get the Fatigue condition. (Sometimes this doesn't happen, in which case you just cast it again.)
Once your main character has the Fatigue condition, save the game. Then cast Contagion. It might take a few tries, but you should get it. If you spend all the spell slots without the main character getting the Disease condition, just reload the save you made.
Once your main character has both the Fatigue and the Disease conditions, save the game again. Next, take Valygar and have him attack your main character with his sword, and the achievement should unlock!
Note 1: Valygar's sword, the "Corthala Family Blade", causes the Bleeding condition, not Poison. But aside from being thematically different, they're both considered just two variants of the same thing. Or in other words, the Bleeding condition is considered the same as having the Poison condition, it just has a different name.
Note 2: This is how I unlocked the achievement, after trying to use the Poison spell many times, with the main character constantly saving against it. (That character did have a high Constitution score though, so if you have a low Constitution character, you could try summoning the druid Cernd to cast the Poison spell. But it's easier to just use Valygar.)