We've also got to remember that Old Valyria was host to a creature called a Firewyrm as well, referenced in A Feast for Crows chapter 22-Arya, regarding Valyria's slave mines, "And there were wyrms in that red darkness too... Firewyrms. Some say they are akin to dragons, for wyrms breathe fire too. Instead of soaring through the sky, they bore through stone and soil. If the old tales can be believed, there were wyrms amongst the Fourteen Flames before the dragons came. The young ones are no larger than that skinny arm of yours(Arya's), but they can grow to monstrous sizes and have no love for men...Burnt and blackened corpses were oft found in shafts where the rocks were cracked or full of holes."
And we know from real life that there are parasitic creatures, worm or not, that can burrow into skin, or are laid in it, which can devour a creature from the inside out, for instance Botfly larvae are laid as eggs on the skin of the host and then burrow beneath the skin to pupate, crawling out of the hole they ate into their host to finish the transition to their adult form after dropping to the ground, but if they're ingested in some manner they can attach themselves to the intestinal lining and grow there.
So if Princess <PERSON> somehow ingested Firewyrm larvae, perhaps, say, through eating the tainted meat of some other creature, and they grew inside her, the larvae would have been absorbing all the nutrients she might have been able to scrounge for which would have been the source of her starvation, their inherently high temperature would have displayed as fever, which would also account for her seemingly roasting from the inside out as they grew, their eating away at her would have accounted for her bleeding, and then, when she died, serpent like creatures with faces, mouths, and, most importantly, hands, crawled out of her, which, if you look at the artwork done by <PERSON> (shown below), describes some of the major points of a Firewyrm, including a face with a beard, mouth, and forearms.
And they died on contact with ice and water, which would be the antithesis to their own natural fire, thereby dousing their own flame.
So perhaps whatever she was infected with was a host of baby Firewyrms.
Alright, so I got asked to edit this to give a bit more information in regards to <PERSON> alleged trip to Valyria, so here’s my answer to that bit. If Princess <PERSON> and <PERSON> traveled to Valeria, <PERSON> infected by Firewyrms, and <PERSON> horribly injured, enough for arguably the strongest dragon of lore to flee, who's to say the two incidents couldn’t be connected. 247
If the Firewyrms had moved above ground at the eruption of the Fourteen Flames and the depopulation of the peninsula, plus the as of now continued eruptions some 400+ years later, who’s to say the pair didn’t somehow stumble into a Firewyrm nest, as I doubt our intrepid princess would want to venture into any cave. And if the Firewyrms are nesting above ground, had <PERSON> managed to cross the Smoking Sea and set foot upon Valyrian soil, who's to say he would not have been attacked as well while exploring the land and collecting Valyrian relics. So no, I doubt he set foot upon Valyrian soil.
But, if he were to figure out a way to survive the no doubt toxic gasses of the Smoking Sea, I wouldn’t doubt he would find a way to trawl the shallows for relics lost in the Shattering of the Valyrian Peninsula during the Doom. That he found the the Dragonbinding Horn and a suit of Valyrian Steel armor while there would be entirely up to speculation, and there are contradictory reports from semi-canonical sources in <PERSON>'s A World of Ice and Fire sidebook, regarding the fact that the horn may have in fact come from a quartet of kidnaped warlocks from <PERSON>, that put the credibility of the horn's Valyrian discovery to question.
And in regards to the Dragon Egg he might have allegedly held at one point or another, <PERSON> himself states in chapter 29 of A Feast for Crows, "I once held a dragon's egg in this hand, brother (referring to <PERSON>). This Myrish wizard swore he could hatch it if I gave him a year and all the gold that he required. When I grew bored with his excuses, I slew him. As he watched his entrails sliding through his fingers he said 'But it has not been a year. 247