Note: The "poison" in the title comes from the Japanese word for syphilis, 梅毒 (literally, plum-poison).
Summary by platina
A pet Reimu argues with her owner, saying that she wants little ones. Mister forbids it.
Mister tells the reader that he bought this cheap Reimu for a purpose other than companionship. At first, he thought of using a stray, but decided that they were too variable/unpredictable--instead, he bought a Reimu suitable for his needs at a yubuse shop.
The next day, he comes home to find Reimu pregnant by a stray Marisa. He reminds Reimu that he told her not to make little ones, but what's done is done, and at the yukkuris' insistence, he adopts the stray Marisa (who is a total shithead, although Mister doesn't seem to mind). Marisa notices that her penipeni stings a little.
Two days later, Mister is awoken by Reimu's screaming. Her little ones have fallen off the stalk, dead. Mister asks Marisa if she feels anything unusual, and--no longer distracted by the dead little ones--she realizes her penipeni hurts badly. Mister explains that Reimu has yuphilis, which will kill her mate and her babies. That's why he told her not to refresh.
Marisa blames Reimu for infecting her. Reimu insists she didn't know about the disease, but Mister lies and says he told her countless times. Marisa tries to kill Reimu, but straining her body causes runny bean paste to gush out of her penipeni. She faints in agony.
Mister tells Reimu that he's disowning her as a pet because she disobeyed him. He tears off her badge and knocks her unconscious with ramune spray.
Mister leaves the two yukkuris in a cardboard box in the overgrown park where Marisa used to live. Mister figures that if scum like Marisa could live there unpunished, the clan must be full of scum. He plans to observe them [via video camera].
Marisa's penipeni has stopped hurting, so Marisa and Reimu have forgotten all about the yuphilis, although they're still angry about being kicked out of their easy place by Mister. They swear they'll get revenge against him, but in the meantime, they gather food by beating up weaker yukkuris.
Mister explains that the penipeni pain is the earliest stage of yuphilis. Reimu was already infected, so she didn't have the pain. The next phase occurs when the disease has spread throughout the body, and since Reimu has had the disease for longer...
When they wake up the next morning, Marisa takes one look at Reimu and runs away peeing and screaming. Reimu's skin is inflamed, oozing and slipping off "like the bad guy in Robocop."
When hungry Reimu realizes that Marisa isn't coming back with food, she starts to go hunting herself, but finds she can't move. When she tries to move, her skin starts sliding off, causing pain.
Reimu is stuck lying forward with her bottom stuck out, and her bottom attracts a passing raper Alice. Reimu isn't raped to death in the usual way--instead, the movement ruptures Reimu's skin and she dies of bean-paste loss. The raper Alice doesn't seem to notice and moves on, refreshed.
Meanwhile, Marisa has run into her old friend Patchouli. Marisa puts the moves on Patchouli, asking her for food and seducing her...
"Mukyuuuuuu!!? Patchy's mamumamu is--!!" ere ere...
"Wh-what's wrong!? Patchouli!!"
"AWIZE'Z ZIDY-ZEGD BENIBENI----!!"
"DIIIIIICK! CHLAMYDIA!! SYPHILIISSS!!?"
"DON'D GED IIIIIID!!"
"Reimu's little ooooones! Take it easyyyyyy!"
"Yupii, yu, eajy!"
The disease quickly spreads throughout the clan, with the raper as the main vector. That particular raper has the habit of holding back and _not_ raping her victims to the point of death (so as not to decrease her number of potential victims.) As a consequence, her victims survive, kill any raper-babies they might have, and go on to find proper mates to refresh with, passing along the disease that will kill their mates and babies. Any babies that are fortunate enough to survive until birth are either retarded or very weak and infected.
If the clan had a wise leader, the clan might be saved, but there aren't any clever yukkuris here. The screams resound through the area for a few days, and when they stop, it's because there are no more yukkuris.
"Yugyaaaaaaa!! Id hurrrrrrtz!?"
"Awize'z zidy-zecd mamumamuuuuu!!"
"Mister bean paste won't stop coming out of [my] penipeniiiii!!?"
"If you don't wan't this to happen--LET'S GET CHECKED!"
"You don't want to make your loved ones suffer."
Mister used his footage to make a commercial for a yukkuri vaccination campaign. He entered it into an amateur commercial contest, and it was broadcast on TV. His disturbing images of yukkuris with their skin inflamed were also used as an illustration evoking smokers' lungs.
However, his video was also popular with abusers, and yuphilis began to be used as a yubuse tool... but that's another story.