I think on the Poison topic, there's also the thought of how hard it is to make the idea of a total immunity to poisons mesh with a design. Pokemon simplifies it down to just the simple concept of "Poison", but realistically there are a large variety of toxic substances or venoms that the Plant and Animal Kingdoms produce, and what works on one species might not work on another (E.G. something that could shrug off Cobra Venom might not have the same luck with a Poison species of Frog). Burns or Paralysis as status-effects have pretty straightforward concepts (residual damage from high heat and scrambled nervous system due to something like Electricity or shock to the system).
Consider that the two natural holders for Immunity are Snorlax and Zangoose. The latter is explained by its Mongoose-basis being a Predator for venomous Snakes, of which Pokemon has a decent few (and besides the universal-Toxic, most non-Poison Typed Snakes don't tend to have very many Poison-inducing moves), while the former's Immunity is linked to it just eating so much stuff indiscriminately (be it poisonous creatures/their secretions, fruit with potentially toxic chemicals, or simply food in unsanitary places like food in Human Garbage cans) that it builds a resistance to toxins so it can continue to eat them. Snorlax is exposed specifically to a very large array of toxic substances as the explanation for its built immunity to just "Poison" as an umbrella.
Gliscor's Poison Heal is a weird topic for me since it was introduced as a Dream World ability (before they simply became the more "mundane" Hidden Abilities), so it might not have been designed as grounded to the same extent as a base ability of a Pokemon. With that said my explanation for Shroomish and Breloom stems from their basis as Mushrooms and their Pokedex entries.
The two Pokemon are described as having highly toxic spores in their biology as a defense mechanism (already alluded to in Effect Spore), and both have a plant based diet from forests with Shroomish's explicitly citing compost from rotting leaves and plant matter (Breloom seems to suggest it simply eats plants in general). Whereas Snorlax and Zangoose need immunity to Poison but don't care if it's present or not, the Shroomish line seems to subsist on Toxic Matter while producing it itself, so its system ultimately benefits from the presence of substances meant to be toxic to other species that it can metabolize and use. Additionally, fungi like Mushrooms are often thought of as thriving in environments where other life dies off (whether by the life cycle or a potential endangerment/mass death event), being particularly common as the catalyst in movies with a plague/zombified type of element to them such as TLOU, so conditions often thought of as toxic to plants and animals can be beneficial to a Fungus, resulting in Poison Heal recovery from a substance that normally hurts most other Pokemon.
The more curious edge-case for me comes from later gens with the Salandit line's "Corrosion" ability, which allows them to inflict Poison on Steel and Poison type targets when using specifically status moves (i.e. Toxic works but Sludge Bomb Proc will not). The name of the ability (and Salazzle's secondary Fire typing) explains the Steel-vulnerability as a more literal idea of melting or corroding the material as a chemical reaction than the internal toxicity we imagine for most poisons, but that still raises the curious question of how Poison Heal Pokemon take that toxin in but still restore their health rather than lost it as a result
Consider that the two natural holders for Immunity are Snorlax and Zangoose. The latter is explained by its Mongoose-basis being a Predator for venomous Snakes, of which Pokemon has a decent few (and besides the universal-Toxic, most non-Poison Typed Snakes don't tend to have very many Poison-inducing moves), while the former's Immunity is linked to it just eating so much stuff indiscriminately (be it poisonous creatures/their secretions, fruit with potentially toxic chemicals, or simply food in unsanitary places like food in Human Garbage cans) that it builds a resistance to toxins so it can continue to eat them. Snorlax is exposed specifically to a very large array of toxic substances as the explanation for its built immunity to just "Poison" as an umbrella.
Gliscor's Poison Heal is a weird topic for me since it was introduced as a Dream World ability (before they simply became the more "mundane" Hidden Abilities), so it might not have been designed as grounded to the same extent as a base ability of a Pokemon. With that said my explanation for Shroomish and Breloom stems from their basis as Mushrooms and their Pokedex entries.
The two Pokemon are described as having highly toxic spores in their biology as a defense mechanism (already alluded to in Effect Spore), and both have a plant based diet from forests with Shroomish's explicitly citing compost from rotting leaves and plant matter (Breloom seems to suggest it simply eats plants in general). Whereas Snorlax and Zangoose need immunity to Poison but don't care if it's present or not, the Shroomish line seems to subsist on Toxic Matter while producing it itself, so its system ultimately benefits from the presence of substances meant to be toxic to other species that it can metabolize and use. Additionally, fungi like Mushrooms are often thought of as thriving in environments where other life dies off (whether by the life cycle or a potential endangerment/mass death event), being particularly common as the catalyst in movies with a plague/zombified type of element to them such as TLOU, so conditions often thought of as toxic to plants and animals can be beneficial to a Fungus, resulting in Poison Heal recovery from a substance that normally hurts most other Pokemon.
The more curious edge-case for me comes from later gens with the Salandit line's "Corrosion" ability, which allows them to inflict Poison on Steel and Poison type targets when using specifically status moves (i.e. Toxic works but Sludge Bomb Proc will not). The name of the ability (and Salazzle's secondary Fire typing) explains the Steel-vulnerability as a more literal idea of melting or corroding the material as a chemical reaction than the internal toxicity we imagine for most poisons, but that still raises the curious question of how Poison Heal Pokemon take that toxin in but still restore their health rather than lost it as a result
