Having your tonsils removed as a child makes you a third more likely to suffer a severe form of arthritis in later life, scientists say.
They also found having older siblings raises the risk by 12% to 34% of ankylosing spondylitis, a long-term condition in which the spine and other areas of the body become inflamed.
The findings bolster the theory that infections in youth trigger the development of the problem.
A Swedish team looked at health and family information in national population registries in order to compare exposure to various early life risk factors.
Patients studied had to have made at least one specialist clinic visit and been diagnosed between 2001 and 2022.
Early risk factors were found to include births falling in either autumn or winter, having serious infections up to the age of 15 and tonsil or appendix removals before the age of 16.
Having two or more older siblings is linked to a 34% greater risk, compared to those with no older brothers or sisters.
But a tonsil removal under the age of 16 was the leading driver of the condition overall, giving a 36% higher risk in cases where other key factors are also at play.
Dr Matilda Morin, of the Karolinska Institutet, said: “The mechanism behind this risk increase cannot be determined from our data.
“But it has been shown that infants with older siblings are more exposed to infections early in life than infants without them.”
