I just found two good reasons for incorporating My First Pet Stuff with my collection of TS4 gameplay:
"If another Sim is already infected with and gets to stage 3 when it’s contagious there’s a chance to spread it when other Sim are near them long enough. It’s a airborne illness so just hanging out with Sim can spread it "
https://sims-online.com/how-to-get-treat-guinea-pig-disease/
"Death by Rabid Rodent Fever: This Sim succumbed to Rabid Rodent Fever, and spreads fear of pestilence and disease in the afterlife. Ghost looks like a giant rodent, foaming at the mouth. Ghost can spread rodent fear which cause nearby Sims to run in fear."
https://sims.fandom.com/wiki/Trait_(The_Sims_4)
That is just so incredibly creepy. Off to create ghosts for my grave yard now.