Parents of young children are being encouraged to prepare for winter and take up the offer of a free flu vaccine.
Under a new immunisation programme, parents with children who were aged two or three years old on September 1 were sent a letter inviting them to receive the nasal spray flu vaccine.
Many have already taken up the offer but NHS Scotland wants about 120,000 two and three-year-olds to be immunised this year, along with 100,000 primary-school pupils, to build a flu defence and reduce the chance of the virus spreading to others.
In total, a fifth of the population will be offered the vaccination, with young children, vulnerable people over 65 and health and social care workers prioritised.
About 2,500 children under the age of five have to go to hospital each year with flu or complications caused by flu, the NHS said.
David Cromie, consultant in public health medicine at NHS Lanarkshire, said: "Flu can be very serious. Even healthy children can become seriously ill from flu and it can spread to others.
"Flu is especially common in winter, so it is important that children receive the flu vaccine before the coldest months of winter.
"Two and three-year-old children are one of the groups most vulnerable to flu as they are unlikely to have built up any protection from previous infections.
"Immunising these children also reduces the chances of spreading an infection to younger children, their parents and grandparents who may themselves be particularly vulnerable to the effects of flu, through a health condition or age.
"I would encourage any parent who has received an invitation for their child to take up the offer of the vaccine as soon as they can."
Flu symptoms include fever, aching muscles and joints, headaches, extreme tiredness and a sore throat which can last for up to a week.
Part of the immunisation programme for primary-school pupils was delayed in Glasgow earlier this month after some Muslim parents raised concerns that the nasal spray contained pork gelatine.
Doctors said it had been agreed by more than 100 Muslim scholars at a World Health Organisation conference in 2001 that the gelatine extract was a completely changed substance by the time it was included in the spray, but an alternative flu vaccine injection was offered to parents who remained concerned.