Objectives:
In this study we investigated the association between autism spectrum disorder in the child, and infections requiring hospitalization in children under the age of 18 months in Denmark.

Methods:
A population based study, using all children born in Denmark from January 1, 1980, through December 31, 2005, identified in the Danish Medical Birth Registry, a total of 1,612,342 children. The cases of autism spectrum disorder were found in the Danish national Psychiatric Register, a total of 7,409 cases. Data on diagnoses of infection was obtained from the Danish National Hospital Register. Data was analyzed using Cox regression model.

Results:
Preliminary results suggested that children admitted in hospital for an infectious disease had a statistically significant higher risk of being diagnosed with autism; these infections included herpes infections, infectious enteritis, pneumonia, upper airway infections, and otitis media. Infections such as conjunctivitis, appendicitis, and tuberculoses did not display a significant association to autism. Children with multiple hospitalizations due to infections displayed even higher risk of being diagnosed with autism.

Conclusions:
There are many possible explanations for the results in this study, 1) these infections could be a part of the etiology of autism, 2) immune response in the child during infection could trigger the development of autism, 3) children with autism could be more susceptible to infections due to an altered immune system, or 4) the hospitalization of these children could have directed the professionals’ attention to developmental deficiencies in the child.