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That child tested negative for the disease, as did a symptomless adult and a symptomless adolescent in the house.

EBV -- a usually inactive, or latent, herpesvirus -- affects most of the human population; more than 90% of people carry the virus as a passive, typically symptomless infection.

The H5N1 avian flu virus has devastated bird populations, and routine testing of farmworkers has found the first, symptomless, cases in people in the UK.

"At any point she could have had a CT scan and it might have been picked up and repaired, but they're symptomless, so there's no way of knowing it was there," said Mrs Taylor.

“I really encourage everyone to have a blood pressure cuff in their house, even a young person” because high blood pressure can be symptomless, McCullough says.