In most cases, a roundworm infection doesn't cause any noticeable symptoms.

When symptoms occur, they usually follow a two-stage course, as described below:

• Early-phase symptoms – these are caused by the larvae (newly-hatched worms) moving from the small intestine to the lungs. It's unclear why some people develop these symptoms while others don't. 
• Late-phase symptoms – these are caused by adult worms living in the intestine. The worms can't reproduce inside you. There will only be as many worms as the eggs you first ingested (see causes of roundworm infection for more details about the life cycle of the roundworm).

Early-phase symptoms

The early-phase symptoms of a roundworm infection only affect a small proportion of people and are rare in the UK.

Symptoms usually begin 4-16 days after swallowing the eggs and last up to three weeks. They include:

• a fever (high temperature) of 38C (100.4F) or above
• a dry cough
• shortness of breath 
• wheezing

Late-phase symptoms

The late-phase symptoms of a roundworm infection are even rarer in the UK, particularly in adults. If there are a lot of mature worms in the intestine, they can cause blockages. This won't occur before six weeks after the eggs have been swallowed.

Signs and symptoms associated with mature worms include:

• passing a worm in your faeces
• mild stomach (abdominal) pain
• nausea and vomiting
• diarrhoea (you may also notice blood in your stools)

However, most people have no symptoms.

If your intestines are blocked due to a large number of worms, you will probably experience vomiting and severe abdominal pain.