Scientists analyzed two different studies in Copenhagen that followed thousands of individuals for at least 10 years. They found that those with very high HDL levels (over 100 mg/dL) had a significantly higher risk of having a bacterial infection, such as bacterial pneumonia or a urinary tract infection, that required a hospital stay. As to why, Borge G. Nordestgaard, M.D., study author and chief physician at Copenhagen University Hospital in Sweden says, “With higher levels of HDL, blood leukocytes [white blood cells] are lower, and blood leukocytes help fight infections.”