The T-Pose Shroom is a poisonous mushroom that turns whoever eats one into a T-Pose Zombie, who then seeks out other people to spread the T-Pose virus to by making direct contact. T-Pose Shrooms are presumably found in the wild, as they are prominent enough in the Mushroom Kingdom for Princess Peach to know about them, but the only ones seen in the SMG4 series were the ones created by Waluigi.
Before the events of SMG4: Mario And The T-Pose Virus, Waluigi approached Shroomy and used the Staff of Rejection to create a T-Pose Shroom, which he gave to Shroomy after commanding him to eat it. After Shroomy ate it (despite the fact that boy scouts should know which mushrooms are safe to eat and which are not), he become a T-Pose Zombie and infected everyone in the castle. They almost infected Mario, but he was rescued at the last second by Princess Peach who, while riding a helicopter, hit all the T-Posers with 1-Up Mushrooms, curing them all.
Once everyone was cured, Peach explained that the T-Pose Virus was caused by a T-Pose Shroom, and then demanded to know who ate a T-Pose Shroom. Everyone was quick to blame Shroomy since he was patient zero of the outbreak. Shroomy confessed and told everyone about how Waluigi gave him the T-Pose Shroom while he was walking through the forest that morning.
Following that, in SMG4: The Mario Convention!, Waluigi, disguised as Hideo Kojima at Glitchcon, attempted to use a T-Pose Shroom on Mario. This T-Pose Shroom was primed to detonate so that it would also infect anyone caught within the blast. Mario was almost caught in the explosion, but Meggy saw the danger and pushed the plump plumber out of the way just before it blew up, sacrificing herself. She became the first victim of the second outbreak.
In SMG4: War Of The Fat Italians 2018, Waluigi removes all the effects caused by T-Pose Mushrooms or infections from all characters who were infected at the convention for the challenges.