Various places/things carry warnings that you should be careful around "children, elderly, and others with weak immune systems."
At what age does a child not fall under this category?
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I'm going to make an educated guess that for many illnesses, the at-risk age is under 5 years old, but children under 2 years of age are particularly vulnerable.
I've garnered this information from several resources.
Influenza is dangerous for children
Influenza (“the flu”) is more dangerous than the common cold for children. Each year, many children get sick with seasonal influenza; some of those illnesses result in death.
- Children commonly need medical care because of influenza, especially before they turn 5 years old.
- Severe influenza complications are most common in children younger than 2 years old.
People at High Risk for Developing Flu-Related Complications
- Children younger than 5, but especially children younger than 2 years old
Another source is a study done "sub-Saharan Africa and south Asia" regarding diarrhea. The study involved over 22,000 children (over half of which were a control group). It found that:
Odds of dying during follow-up were 8·5-fold higher in patients with moderate-to-severe diarrhoea than in controls (odd ratio 8·5, 95% CI 5·8–12·5, p<0·0001); most deaths (167 [87·9%]) occurred during the first 2 years of life
Another source for the 5-year mark, rather than the 2-year mark, is a Polio awareness website:
Who is at risk?
Polio can strike at any age, but it mainly affects children under five years old.
The World Health Organization specifically tracks rotavirus-related deaths for children aged 5 and under.
The case-fatality rate for diphtheria has changed very little during the last 50 years. The overall case-fatality rate for diphtheria is 5%–10%, with higher death rates (up to 20%) among persons younger than 5 and older than 40 years of age. Before there was treatment for diphtheria, the disease was fatal in up to half of cases.
Deaths from measles primarily occur in children under 5.
The disease remains one of the leading causes of death among young children globally, despite the availability of a safe and effective vaccine. Approximately 145 700 people died from measles in 2013 – mostly children under the age of 5.
I intentionally sought data on the deadliest childhood diseases, rather than some other common illness such as the cold or ear infections.
One of the reasons I did this is because the deadly diseases are under more heavy surveillance by governing bodies of disease prevention, such as the CDC and WHO.
Another reason is that I believe the increased rates of death could be extrapolated to mean decreased immune strength. I can't back up this second belief with any evidence, and I know there are other possible contributing factors: a younger body has different physiology other than just weaker immune systems.
However, I would be comfortable in assuming that if these terrible illness wreak more havoc in children under 5 years old, then less horrible illnesses are also likely to be more severe for the same age group.
Note: Most of the illnesses I've mentioned are preventable through vaccination. Deaths are more commonly found in populations without access to vaccinations.