# Daycare illnesses

Before I had a baby I was pretty agnostic about the idea of daycare. I could imagine various pros and cons but I didn’t have a strong overall opinion. Then I started mentioning the idea to various people. Every parent I spoke to brought up a consideration I hadn’t thought about before—the illnesses.

A number of parents, including family members, told me they had sent their baby to daycare only for them to become constantly ill, sometimes severely, until they decided to take them out. This worried me so I asked around some more. Invariably *every single parent *who had tried to send their babies or toddlers to daycare, or who had babies in daycare right now, told me that they were ill more often than not.

One mother strongly advised me never to send my baby to daycare. She regretted sending her (normal and healthy) first son to daycare when he was one—he ended up hospitalized with severe pneumonia after a few months of constant illnesses and infections. She told me that after that she didn’t send her other kids to daycare and they had much healthier childhoods.

I also started paying more attention to the kids I saw playing outside with their daycare group and noticing that every one had a sniffly nose.

I asked on a mothers group chat about people’s experiences with daycare. Again, the same. Some quotes:

“They do get sick a lot. I started my son at 2.5 and feel he always has something.”

“The limit does not exist.”

“brought home every plague (in first 6mo, Covid, HFM, slapcheek, RSV)”

“They usually say 8-12 illnesses per year. My girls were sick every 2-3 weeks in their first year of daycare”

“My daughter started daycare at 6 months and got sick a ton the first year”

Despite all this, many parents who have the option not to (i.e. they can afford in-home care with a nanny or for one parent to stay home) still choose to send their babies and toddlers to daycare. How come? Surely most well-off adults wouldn’t agree to be ill nonstop in exchange for the monetary savings daycare provides?

Asking around, it seemed like the most common reason given was that parents believed daycare illnesses “built immunity”; that if their babies and toddlers got sick at daycare they’d get less sick later in childhood and so overall it would net out the same. Unfortunately few could point me to any evidence for this but nevertheless passionately defended the view.

The claim that daycare illnesses simply offset childhood and adult illness immediately seemed suspect to me for a number of reasons:

1.  (Quite confident) The most common illnesses (colds and flu) don’t build immunity in general (in kids or adults) because they mutate every year
2.  (Quite confident) The same illness has a greater risk of complications in babies vs. older children and adults
3.  (Moderately confident) The same illness has a greater duration in babies vs. older children and adults
4.  (Moderately confident) Illness during early development is probably more harmful than illness during adulthood
5.  (Weak guess) Daycare environments are more conducive to disease spread than schools for older kids and the number of possible illnesses is very high; there isn’t just a limited number of things you catch once

I [xeeted](https://x.com/NinaPanickssery/status/2038279892073349144?s=20) about this:

[![](https://substackcdn.com/image/fetch/$s_!O46q!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Fe49ffb69-1bac-4f37-96e2-e4cdd30bb3e7_1168x294.png)](https://substackcdn.com/image/fetch/$s_!O46q!,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Fe49ffb69-1bac-4f37-96e2-e4cdd30bb3e7_1168x294.png)

A number of people sent me [this link](https://www.ucl.ac.uk/news/2026/mar/frequent-infections-nursery-help-toddlers-build-immune-systems), an alleged “study” from UCL showing that “frequent infections in nursery help toddlers build up immune systems”, authored (of course) by a group of parents who all send their kids to nursery (what the British call daycare).

The link I was sent was actually a UCL press release summarizing a [narrative review paper](https://journals.asm.org/doi/10.1128/cmr.00253-25) and not a study itself. Narrative reviews are susceptible to selection bias because, unlike systematic reviews or meta-analyses, there’s no pre-registered search protocol or [PRISMA](https://www.prisma-statement.org/)-style methodology requiring them to account for all relevant evidence. But I decided to look into the narrative review more, to assess its validity fairly. I got access to the full publication.

Unlike the press release, which ignores these considerations entirely, it *does* engage with severity and age-related vulnerability, conceding that younger toddlers and babies suffer more from the same illnesses. A section on immunology provides a detailed account of why infants under two are more vulnerable—their immune systems are much less effective at fighting the same infections for a plethora of well-understood reasons. The review also cites a large Danish registry study ([Kamper-Jørgensen et al](https://publications.aap.org/pediatrics/article-abstract/118/4/1439/69092/Population-Based-Study-of-the-Impact-of-Childcare)) that reports a 69% higher incidence of hospitalization for acute respiratory infections in under-1s in daycare.

However, these severity findings are integrated into the review’s conclusions and framing in an incredibly biased way. The introduction describes severe outcomes as occurring “in rare cases,” and the conclusions focus on normalizing the burden and advocating for employer understanding. After establishing the immunological basis for why the same infection is more dangerous in a 6-month-old than a 3-year-old, it doesn’t then ask the hard follow-up question: given this, is the pattern of starting daycare at 6–12 months optimal from a child health perspective? Instead, the review frames this timing as a societal given. The Hand Foot and Mouth Disease section is a good example of the review’s handling: it reports that daycare attendance was associated with more severe cases but then immediately offers mitigating interpretation with no evidence—that prolonged hospital stays might reflect parental work constraints rather than genuine severity.

[![](https://substackcdn.com/image/fetch/$s_!no9q!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Fc211b991-ff6c-490c-8372-5858a6a2e45b_1258x1098.png)](https://substackcdn.com/image/fetch/$s_!no9q!,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Fc211b991-ff6c-490c-8372-5858a6a2e45b_1258x1098.png)

Though the review considers severity, it ignores duration. Their primary metric throughout is episode count. Also, despite discussing a wide variety of pathogens, it doesn’t address which of these infections carry the highest complication rates in infants and toddlers specifically.

Finally, the crucial “Illness now or illness later?” is the paper’s weakest portion. It rests on two primary sources for the compensatory immunity claim:

*   The Tucson Children’s Respiratory Study: a cohort study of ~1,000 American children followed from birth to age 13 in the early 2000s, finding that daycare attendees had more colds at age 2 but fewer by age 6–11.
*   A Dutch study (Hullegie et al. 2016) of 2220 children followed for 6 years, finding reduced GI illness between ages 2–5 in children with first-year daycare attendance.

These are reasonable small studies, but the paper does not cite or engage with the [Søegaard et al. 2023 study (International Journal of Epidemiology)](https://academic.oup.com/ije/article/52/2/466/6839854)—a register-based cohort of over 1 million Danish children followed to age 20, which directly tested and rejected the compensatory immunity hypothesis. Quoting from the study:

> We observed 4 599 993 independent episodes of infection (antimicrobial exposure) during follow-up. Childcare enrolment transiently increased infection rates; the younger the child, the greater the increase. The resulting increased cumulative number of infections associated with earlier age at childcare enrolment was not compensated by lower infection risk later in childhood or adolescence.

This is arguably the single most relevant study for the paper’s central “illness now or illness later” question, and it’s three orders of magnitude larger than either study the authors cite. Its absence is hard to explain—it was published in a top epidemiology journal in late 2022 (available online November 2022), well before the review was written.

Accordingly, they hedge their conclusions carefully—“attendance at formal childcare may tip the balance in favor of infection now rather than later”, but their press release ignores any nuance, referring to daycare as an “immune boot camp”.

So overall, the compensatory immunity claim seems very weak and my prior that daycare illness is straight-up bad remains. Parents are citing biased reviews from motivated researchers. We are only [beginning to understand the deleterious effects of increased viral load in infants](https://www.nber.org/system/files/working_papers/w29524/w29524.pdf).

[![](https://substackcdn.com/image/fetch/$s_!sdIw!,w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Fa2c2a4aa-a501-4cd5-a3ac-80014b01c740_1168x1210.png)](https://substackcdn.com/image/fetch/$s_!sdIw!,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Fa2c2a4aa-a501-4cd5-a3ac-80014b01c740_1168x1210.png)

I predict that in the future we’ll learn more about the side-effects of increased viral load on intelligence, wellbeing, fatigue etc. The “just the sniffles” mentality is a harmful attitude toward infections that promotes the dismissal of phenomena that substantially impact child and adult wellbeing.