Worms in humans are caused primarily by the ingestion of worm eggs or larvae, which can enter the body through contaminated food, water, soil, or contact with infected feces. Worms are parasites that survive by living in the human intestine or other tissues, feeding off the host. Common ways people get worms include touching contaminated surfaces or objects, eating undercooked or raw meat from infected animals, and coming into contact with contaminated soil or feces from humans or animals. Some specific causes are:

• Eating food or drinking water contaminated with worm eggs, such as unwashed vegetables or undercooked meat (including pork, wild game, or fish) infected with worms like roundworms or Trichinella (which cause trichinosis).

• Swallowing microscopic eggs from surfaces contaminated by infected people's feces, especially common with threadworms (pinworms), often spread via hand-to-mouth contact after touching bed linen, bathroom surfaces, or contaminated objects.

• Contact with soil contaminated with feces containing worm eggs, which can be ingested accidentally, especially by children playing outdoors.

• In some regions, poor sanitation and the use of human or pig feces as fertilizer can increase the risk of infection with soil-transmitted worms.

• Contact with animal feces contaminated with worm eggs, such as from dogs or cats, can lead to infections like toxocariasis.

Overall, worms are spread through fecal-oral transmission routes or by eating contaminated food, and poor hygiene, sanitation, and inadequate cooking of meats are major contributing factors to worm infections in humans.