Pupils in about 60 schools across north-east England have been sent home due to symptoms of flu and norovirus.
Public Health England said it could not give an exact figure for the number of schools affected or where they were.
Some schools in the region have closed to undergo a "deep clean".
Public Health England said it expects illnesses of this kind to circulate in schools and the community at this time of year and the flu vaccination is the best form of protection.
Dr Gayle Dolan from Public Health North East said: "If you are well continue activities as normal, but if you are unwell it is important to stay away from school or work until you are better.
"If you have diarrhoea or vomiting that is 48 hours after you have had symptoms and if you have flu it is until you are feeling better."
People have been encouraged to wash hands thoroughly and regularly, particularly after using the toilet and before eating.
What is norovirus?
Sometimes known as "winter vomiting viruses" noroviruses are the most common cause of stomach bugs in the UK, affecting all ages.
It is estimated that between 600,000 and one million people in the UK become infected each year.
About 12 to 48 hours after becoming infected, the virus causes sudden onset of nausea followed by vomiting and diarrhoea.
The illness is not generally dangerous and most people make a full recovery in one or two days.