Dr Lucy van Dorp (UCL Genetics Institute), along with a colleague, explains in an article for The Conversation how illnesses contracted by toddlers at nursery can help strengthen their immune systems in the long term.
There’s no nice way to put it: small children are snotty. A research study that tested children for multiple respiratory viruses every week for a year found that under-fives are carrying one or more viruses 50% of the time. A child aged 15 months will have 12-15 colds per year and eight or nine of those will show symptoms, such as a runny nose. If parents feel their small children are sick with a cold half the time, that’s backed up by evidence.
Our new study brings together information from different sources to understand how the immune systems of small children differ from adults, why children pass germs around so effectively and what parents can do to help.
Babies and toddlers are encountering a whole host of infectious diseases for the first time, which makes them more likely to have symptoms; this can make them more likely to pass these bugs on to others. We also found that going to nursery often accelerates a natural process of building up immunity that would otherwise take place at school. Finally, we highlighted vaccination as a major tool to reduce childhood sick days.
It’s worth saying that most of the evidence we reviewed is from studies conducted before COVID showed up. We don’t yet know whether COVID adds another virus to this mix.
Children experience about twice as many bouts of diarrhoea and vomiting per year as adults (two on average). Having an under five in your household increases your risk as an adult of having diarrhoea and vomiting three- to fivefold.
Children also pick up multiple viruses and bacteria that can cause rashes or skin infections. By 12 months of age, 70% of children have antibodies to the two viruses (HHV6A and 6B) that cause most cases of roseola (a common infection, usually causing high temperature and a rash). By age five, 65% of children in the UK have antibodies to varicella-zoster virus (chickenpox). In short, twelve colds, two rounds of diarrhoea and vomiting and one or two rash illnesses is typical between 12 and 24 months of age, with or without nursery.
Nursery bugs
But parents aren’t exaggerating when they say that starting at nursery leads to children and infants passing more germs around than might be caught at a birthday party or baby class. The process of picking up germs that cause colds, stomach bugs and rashes is sped up by starting at nursery and continues for one to two years.
However, this is a trade off against disease later.
Studies have shown that children who attend nursery have fewer infections when they start primary school than children who did not attend nursery. The idea that nursery gets bugs out of the way before starting school has evidence behind it. Whether or not this is a good thing may come down to parents’ opinions.
In some cases, delaying the age children get an infection for the first time is beneficial , such as for RSV, the cause of bronchiolitis. For others, earlier infection or vaccination (such as for varicella-zoster virus, the cause of chickenpox) seems to lead to milder symptoms than if infection is delayed to later in childhood.
We think it’s important for parents’ employers to know that all small children get ill frequently, and that this is entirely normal - their immune systems are barraged with new infections and they must develop immunity to those infections that can’t be prevented with vaccination. It’s not an indicator of poor hygiene at a nursery or parents being precious. Children can transmit before and after they show symptoms, simply keeping ill children home isn’t enough to stop transmission.
Beyond better support from employers, how do we make this situation better for young children and their parents and carers? We found in our research that vaccination is one of the most effective things we can do to prevent children from becoming sick and to reduce their symptoms.
With large outbreaks of measles in the US and UK, ensuring that young children receive their recommended vaccines, such as MMRV (for measles, mumps, rubella and, since early 2026, chickenpox) has never been more important. Between January 1 and March 6 2026, 34% of England’s measles cases were in the one to four years (nursery) age group. In summer 2025, over 96% of measles cases were in unvaccinated people.
In the future, vaccination programmes could even be designed to delay infection to the age at which it is safest for children to contract each specific infection, based on whether they are likely to be exposed at nursery.
Finally, we want to reassure parents that age, better hand hygiene and a more experienced immune system mean that rates of illness for nursery children go down by 50% each year of attendance. Things can only get better.
*This article was originally published in The Conversation on March 18, 2026.
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