Although influenza is usually a self-limited disease, patients who develop 
complications are at increased risk of hospitalization, intensive care unit 
admission and death. Since preventive and early therapeutic measures should be 
prioritized in higher risk patients, identification of the risk factors for 
severe infection is important from a public health perspective. Risk factors for 
complications in pandemics may show some differences with regard to seasonal 
influenza. During the influenza A(H1N1)pmd09 pandemic, although many cases 
occurred in younger adults, the risk factors identified for severe infections 
and complications were similar to those for seasonal influenza, including 
chronic respiratory, renal, liver, and heart diseases. Aged patients, although 
less frequently affected, were also at higher risk. Obesity, and particularly 
morbid obesity (>40 body mass index) has been noted as a significant risk factor 
for severe disease in the 2009 influenza pandemic. Some interesting recent 
studies provide insights into the biological reasons behind the poor outcomes in 
morbidly obese patients. In terms of pregnancy, the studies have shown 
contradictory results due to variations in methodology and medical care. 
However, it seems that pregnancy, particularly during the third trimester, 
increases the risk of complications, and that early antiviral treatment is 
associated with improved outcomes.