Ascariasis: This is the most common intestinal worm infection worldwide. The presence of a worm in vomit is a classic symptom, especially in children, as the worm can migrate up the gastrointestinal tract and be regurgitated.

Hookworm infection: While less common than ascariasis, hookworms can also cause gastrointestinal symptoms and potentially be vomited up, especially in severe infections.
Roundworm (Toxocara) infection: Though more commonly associated with visceral or ocular larva migrans, in rare cases, Toxocara can cause gastrointestinal symptoms, including vomiting, where a worm might be seen.

Anisakiasis: Caused by the ingestion of undercooked or raw fish containing Anisakis larvae, this infection can lead to severe gastrointestinal symptoms, including vomiting. Although less common, it's critical to consider due to its potential for severe allergic reactions and anaphylaxis.
Intestinal obstruction by a worm: In rare cases, a large worm like Ascaris can cause an intestinal obstruction, which is a medical emergency. Vomiting a worm could be a sign of this condition.

Tapeworm (Taenia spp.) infection: While tapeworms are typically associated with intestinal infections and are usually diagnosed by finding segments (proglottids) in the stool, it's theoretically possible for a piece of a tapeworm to be vomited, though this would be extremely rare.
Gnathostomiasis: This is a parasitic infection caused by Gnathostoma spp., typically acquired through eating undercooked fish. It can cause a variety of symptoms, including gastrointestinal issues, but vomiting a worm would be an unusual presentation.