A recent study by the Centers for Disease Control and Prevention (CDC) found that of the 36 children who died from H1N1 from April to August, six had no chronic health conditions. But all of them had a co-occurring bacterial infection.

The most common co-occurring infection that causes flu-related deaths is staphylococcus aureus. A third of the population carries it, most in their nose or on their skin.

The flu causes upper respiratory damage, which allows the staph to make its way into the lungs.