Parents in England are being urged to keep children off school if they are unwell and have a fever, amid high levels of flu and Covid-19 cases.
The same applies for nurseries, according to advice from the UK Health Security Agency (UKHSA).
The number of scarlet fever cases is also high, it warned.
A head teachers' union welcomed the advice, issued as pupils prepare to return to lessons after the Christmas holidays.
Flu and Covid cases are "currently circulating at high levels and are likely to continue to increase in coming weeks", the UKHSA said.
"High numbers of scarlet fever, which is caused by group A streptococcus, also continue to be reported."
Prof Susan Hopkins, its chief medical adviser, said: "If your child is unwell and has a fever, they should stay home from school or nursery until they feel better and the fever has resolved."
She also stressed the importance of washing hands and catching coughs and sneezes in tissues.
Adults should also stay at home when unwell and wear a face covering if they have to go out, she said. Those who are ill should avoid healthcare settings and vulnerable people unless urgent, she added.
Prof Hopkins said uptake of the flu vaccine remained low among young children, but it was still available for:
- children aged two and three on 31 August
- all primary school children
- some secondary school children
More than 1.4 million people in the UK, about one-in-45, were infected with Covid in the week ending 9 December, according to the latest official data.
Hospital admissions from flu in England were at their highest level since the winter of 2017-2018.
Health officials also say parents should be aware of strep A, an infection that can cause scarlet fever, after children in the UK died from it.
Absence rates across schools in England rose sharply at the start of December as more pupils missed class due to illness.
The proportion of children off sick rose to 7.5% in the week commencing 5 December - up from 6.1% the previous week and 2.6% at the start of the term.
Overall absence rates up to that week surpassed the whole of the 2021 autumn term, when Omicron was taking hold.
James Bowen, director of policy for school leaders' union NAHT, said there "does appear to be an unusually high level of illness around at the moment, even for this time of year".
"Advice from government is welcome to give schools and parents clarity on when children should stay at home," he said.
"It is quite common for school policies to already state that children with a fever should remain at home, so this shouldn't represent a major departure from existing policies."