Glynn et al conducted a systematic review to evaluate the severity by age of 32 infectious diseases. After reviewing 19 viral and 13 bacterial diseases, the researchers determined that disease severity was lowest in school-age children for cases of polio, typhoid, tuberculosis, HIV, Ebola, meningococcal meningitis, and influenza, among others. The researchers proposed that adults may face higher exposure risks through behaviors such as caring responsibilities, which exposes them to higher pathogen doses. Furthermore, comorbidities have a higher prevalence in adult populations compared to school-aged children, further contributing to higher case fatality rates. Infants with weak immune systems quickly develop a strong adaptive immune system once born, but this adaptive immune system begins to decline with old age. Puberty and its associated hormone changes were also considered contributing factors to increased case fatality rates with increased age. The researchers proposed that immune function peaks between 5-14 years, and that understanding the age of lowest severity for specific diseases can help determine vaccine schedules and drug design in the future.