The dreaded chickenpox - highly contagious and unmistakable with its itchy rash, starting as small red spots which develop into fluid-filled blisters and eventually scabs. There may be mild flu-like symptoms beforehand too. It's most common in children under 10, and the severity of the rash can range from just a few spots to covering most of the body.
While unsightly and extremely itchy, for most kids it isn't serious, and lots of parents hope to get it out of the way sooner rather than later.
"In pre-school children, chickenpox is a good illness to get because you're immune to it afterwards and it doesn't cause a lot of trouble," says Macgregor.
"If you've got a generally healthy child, chickenpox is a bother and can be disruptive. It's not usually a terrible illness but it can cause a lot of problems for children and adults who have immune problems."
It can also cause pregnancy complications, although this is rare, because most women will have had the virus as children.
Measles, mumps and rubella
These three highly infectious conditions are usually mild, but can have serious, potentially fatal, complications, including meningitis, swelling of the brain (encephalitis) and deafness.
They can also lead to pregnancy complications, and mumps - characterised by painful swelling at the sides of the face - can spread to the testes in post-pubescent males, which can lead to infertility.
Since the MMR vaccine was introduced in 1988, it's rare for children in the UK to catch the illnesses, although there have been measles outbreaks in recent years.
Dr Macgregor urges any parents whose children haven't had the MMR jab to get them vaccinated immediately. Although there was a catch-up programme after the MMR vaccination scare in the late Nineties, some children missed out on having the jab.