Microscopic roundworm eggs are shed into the environment in faeces by infected animals, where they can survive for up to three years (long after the faeces has disappeared).
These eggs are then swallowed by other animals as they graze or snuffle around in the grass. Cats and dogs can become infected this way, or if they eat an infected rodent.
Inside the infected animal, eggs then develop into adult worms, which shed more eggs, thus continuing the life cycle.
Humans can inadvertently swallow roundworm eggs if they get them on their hands and, say, handle food. Children are thought to be more at risk because they are more likely to lick their hands without washing.
If a farm animal ingests roundworm, eggs can be present in their meat - another source of human infection if not cooked properly.
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