What happens when a PC being infested with Rot Grubs chooses to Wild Shape into a beast?

Question

Some context: We're running the Dragon Heist campaign and had an initially boring "sewers crawl" encounter that I've decided to "spice up" by dropping a bunch of Rot Grubs on characters heads (Rot Grubs are super scary insects that bite through person's skin and then "dig a tunnel to the person's heart" and instantly kill the person if they get to the heart 😱) Especially I dropped a bit more Rot Grubs on the druid PC because she was a super timid girl who loves nature but is terrified of insects 😁.

This all initially played extremely well and was quite an awesome emotional experience for the whole party and especially for the druid player... And then the Rot Grubs-infested druid suddenly decides to Wild Shape into an alligator...

As a new DM I was a bit stunned by this and didn't know how to deal with it...

One opinion: One of the party members suggested that the Rot Grubs will just drop on the floor, because they are an "alien" object to the druid PC and hence they will NOT "temporarily vanish" like the other druid's equipment when she's wild shaping. His logic was that when the Wild Shape happens, the druid basically disappears for a second with all of her OWN stuff and reappears at a different location nearby in a form of an animal. And Rot Grubs are NOT druid's "own stuff" so they'll just be at the same spot in space where they were when being in the druid's body and they will just drop on the floor... And so they will NOT be "teleported" to the new druid form... His other rationale was "OK, but what if she transforms into a tiny spider that is smaller than a single rot grub?"

Another opinion: The other guy said that the Rot Grubs, after biting into the body are now basically like a poison inside the druid's body and so they should continue to harm the PC in it's new form. He said that Wild Shape is NOT supposed to be a cure for poison or other diseases (when Rot Grubs are inside the body, you can cure them with the Cure Disease spell). The counter-argument was that the Rot Grubs are still actually creatures, even when in the human's body. They even have their "turns" while being inside the body! So they are not a disease, but a creature that attacks you and even continues to live and tries to infest other victims after killing the original victim...

I end up ruling that the Rot Grubs will magically disappear while the druid is still in the Wild Shape and that they will magically reappear even after a half an hour, after the druid transforms back to her infested human body form (to be immediately cured by the paladin). Actually, I really liked that my party came up with this unexpected move and this was quite a healthy and fun argument! Yet I'm still not sure if that was the most wise ruling, and also back then I spent like 5 minutes goolging for an expert suggestion on this and I wish I would've found it somewhere and haven't damaged the vibe of that nail-biting battle for the players...

Here are my questions:

1. What should happen to a monster that is "inside" a druid that transforms into a larger Wild Shape? (the monster being The Swarm of Rot Grubs or any other creature that digs into the person's body)
2. What if a chosen Wild Shape is not a large beast like an alligator but a tiny spider, smaller than the Rot Grubs or other infecting creature that was inside the person?
3. What if the monsters are not "inside" the druid's body but are just strongly attached to the body with their sharp teeth? Or what about the Slithering Tracker? Will these monsters still be attached to the alligator's body? (or would the Wild Shape transformation process "shake everyone away"?)

Answer 1

The rules are not covering this, your call as a DM

The Wild Shape rules do not cover parasitic infestations. Thus, you are left with the guidance from Tasha's Cauldron of Everything, p. 4:

The rules of D&D cover many of the twists and turns that come up in play, but the possibilities are so vast that the rules can't cover everything. When you encounter something that the rules don't cover or if you're unsure how to interpret a rule, the DM decides how to proceed, aiming for a course that brings the most enjoyment to your whole group.

The rest of this answer is exploring the various options, but this is what it ultimately comes down to.

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The lore in the Rot Grub entry from Monsters of the Multiverse states (emphasis mine):

Any creature that comes into contact with a single rot grub must succeed on a DC 10 Constitution saving throw or be poisoned, as the rot grub burrows into the creature. The poisoned creature takes 3 (1d6) poison damage at the end of each of its turns. Whenever the poisoned creature takes fire damage, the creature can repeat the saving throw, ending the effect on itself on a success. If the poisoned creature ends its turn with 0 hit points, it dies, as the rot grub kills it.

Treating it as poison

The swarm of rot grubs induces the same mechanism (along with 2d6 piercing damage) on a hit, although the wording is slightly different: it leaves out the mentioning of rot grubs italicized above. So, mechanically it is just a poison effect.

Normally conditions, diseases and poisons are not affected by Wild Shape.

If you opt to treat the effect as poison, the damage will continue after Wild Shape. Once the beast form is reduced to 0 hp, the druid will revert to her original form. Turning into a Tiny creature is not different when you treat it like this.

(As an aside, they revert immediately when they hit 0 hp; since they would only die at the end of the turn while infested with rot grubs, they revert before they would die. Only if spillover or continued poison damage reduces their original form to 0 HP will they actually die.)

Treating it as a burrowed creature

It is clear from the lore that the poison damage is caused by at least one rot grub burrowing in the host creature. The poison damage is just a mechanism to track the grub burrowing to the heart and eventually killing the host, because the game has no general rules mechanism for parasitic infestations.

The Wild Shape rules likewise have nothing on how to handle parasites. The closest is the rule on treating equipment, so you can take guidance from that how to think about it (even though the grub is not technically equipment):

You choose whether your equipment falls to the ground in your space, merges into your new form, or is worn by it. Worn equipment functions as normal, but the DM decides whether it is practical for the new form to wear a piece of equipment, based on the creature’s shape and size. Your equipment doesn’t change size or shape to match the new form, and any equipment that the new form can’t wear must either fall to the ground or merge with it. Equipment that merges with the form has no effect until you leave the form.

In this case, "wearing" would mean keeping the grubs in and letting the effect continue, "dropping" would drop them to the ground together with other equipment, and "merging" would make them have no effect until the druid left her beast form. If the new form would be too tiny for the grub, and you choose "wearing" the grub would drop to the ground.

Attached creatures

The grubs are not attached, they are burrowing inside the target. This does not apply to them.

The Slithering Tracker causes the grappled condition, and Wild Shape does not end the grappled condition, so the tracker would continue to affect the beast form.

For other creatures this is probably evaluated best case by case. A Stirge, for example, is just attached; it is not grappling its target. I think that like with other conditions, Wild Shape does not mention anything about affecting attached creatures or "shaking loose", so by default, they would remain attached.

Answer 2

See the issue is, it's magic...

Ultimately, It's up to you how to rule it.

1. What should happen to a monster that is "inside" a druid that transforms into a larger Wild Shape?

I would clarify as to why the player made this choice - if they were increasing their size to make the monster inside less of an impact, perhaps if the monster is eating their way in,. is halfway to the bone, they increase their size and now the monster has to go twice as far.

As for the "merging" of other creatures, based on the description of their equipment (emphasis mine:)

• You choose whether your Equipment falls to the ground in your space, merges into your new form, or is worn by it. Worn Equipment functions as normal, but the DM decides whether it is practical for the new form to wear a piece of Equipment, based on the creature's shape and size. Your Equipment doesn't Change Size or shape to match the new form, and any Equipment that the new form can't wear must either fall to the ground or merge with it. Equipment that merges with the form has no Effect until you leave the form.

Now, this is specifically the Druid's equipment, but it could give guidance as to your choice as the DM as to how to rule this.

2. What if a chosen Wild Shape is not a large beast like an alligator but a tiny spider, smaller than the Rot Grubs or other infecting creature that was inside the person?

This could go one of two ways: one, the larger creature is forced out of the small one, physically and forcefully, both targets taking damage. An example of this was when a Druid was consumed by an Ogre, than Wild Shaped into a Bear, the DM ruled that (particularly since the Ogre was low on health), this finished the encounter rather spectacularly. If you go this route, I would advise the player reconsider their choice, or limit the size of the transformation.

Or two: they are magically ejected, and now the tiny Wild Shaped creature has to deal with creatures that are as big, if not bigger than it. This could create quite the interaction, as a swarm of them dealing with the rest of the party would be highly distracted, causing not only the interaction with a swarm of now Large Rot Grubs, but also the party would unlikely be able to identify the spider among them, causing extra dangers.

3. Will these monsters still be attached to the alligator's body? (or would the Wild Shape transformation process "shake everyone away"?)

[Warning: Incoming Personal Opinion]

Personally, I would rule that they would still be attached, if the target was big enough to grip to still. E.g. if they transformed into a tiny creature, they would likely be small enough to at least wrestle themselves free of the attacking creature's mouths or grip. Again, bouncing off the Player's intent behind the transformation should be the best way forward - a Large Alligator would have a higher AC, but then having more body to attach to would likely also gain more damage (more of them attacking at once).

Ultimately, as the DM, using the magic logic can really help to manage situations like this, but the main thing you want to try to achieve is a fun experience. Players coming up with inventive or creative solutions should be awarded, but at the same time you can also add other consequences if you wish to keep things interesting, and keep the intensity high..