Scientists have identified a number of "red flag" warning signs to help doctors pinpoint serious infections in children.
Serious infections are rare in developed countries and doctors find it hard to diagnose youngsters with conditions such as meningitis and sepis, the scientists said.
Writing in The Lancet medical journal, they said symptoms including a temperature over 40C, quick breathing and rashes all pointed to a child suffering from more than a cough or cold.
Co-author Dr Matthew Thompson, from Oxford University, said: "For doctors, it's a bit like finding a needle in a haystack. Serious infections are rare and getting increasingly rarer thanks to vaccinations. Identifying that one child out of all those many with minor ailments is difficult. It is complicated further as the child may be seen at any early stage of infection before it is possible to recognise its severity."
The researchers now want to develop guidelines for GPs so that children with infections are referred to hospitals quickly and only when necessary.
Dr Ann Van den Bruel, another member of the Oxford team, said: "Doctors should routinely check for these warning signs in every sick child they see. For example, not all GPs will check a child's temperature, whereas we would now suggest this is done on all occasions."