A child who starts nursery often seems to be continuously unwell for the first year or two. The pattern is expected, and the reason is arithmetic rather than fragility.
Immunity is built one virus type at a time
The common cold is not one virus. Rhinoviruses alone exist in a large number of distinct types, and several other virus families produce similar illness.
Recovering from one type produces reasonable protection against that type and very little against the others. Each new type therefore behaves like a first infection.
An adult has accumulated exposure over decades and meets fewer novel types. A three-year-old has met almost none, so nearly every encounter is a new one.
Nursery concentrates exposure
Group settings put many partially immune children in shared air and shared contact for long periods. Any virus entering the group has an unusually efficient route through it.
Small children also shed virus for longer than adults and at higher levels, and they have no meaningful control over hand contact, mouthing objects or coughing direction.
The result is that a child's exposure rate rises abruptly on starting nursery or school, and the frequency of illness rises with it before settling in later years.
The infections often run into each other
Because the interval between exposures is short, a second infection can begin before the first has fully resolved. To a parent this reads as one illness lasting a month.
Cough in particular can persist for weeks after the infection itself has cleared, since airway irritation resolves more slowly than the virus does.
Distinguishing a prolonged single illness from consecutive separate ones is difficult at home, which is one reason parents overestimate how unusual their child's pattern is.
Timing shifts, the total does not shift as much
Children who start group care later tend to have their run of infections later rather than avoiding it. The exposure is deferred to the first school years instead.
Household composition also matters. Younger siblings of school-age children meet the same viruses at home, often earlier and without ever attending a nursery.
This is why comparisons between families are unhelpful. The relevant variable is when a child first joined a large mixing group, not how robust they are.
Where the pattern stops being ordinary
Frequent mild respiratory infections in a child who recovers fully between them, grows normally and is otherwise well is the expected picture at this age.
Different questions arise where infections are unusually severe, involve deeper sites repeatedly, fail to clear, or are accompanied by poor weight gain or persistent tiredness.
Those are the circumstances in which a doctor should assess the pattern rather than the individual illness, and a record of dates and symptoms is more useful than a general impression.