Nearly half of us could be infected with a virus which makes us more stupid, scientists have found.
The startling discovery suggests that millions may be carrying a long-lasting infection which dulls the brain.
Scientists found the virus living in the throats of 44 per cent of patients tested in a small US study.
Those who were carrying the infection performed worse in intelligence tests, even when education and age were taken into account.
The virus – called chlorovirus ATCV-1 – was previously only known to appear in green algae in freshwater lakes.
The researchers, from Johns Hopkins University in Baltimore and the University of Nebraska, have not established how the virus comes to infect humans.