'Bad' Digestive bacteria (Like Staph Aureus) cause the conversion of HDL to LDL, and it probably either does this when creating the biofilms on the vessel wall (When the immune system pulls calcium and cholesterol to contain the staph infection, the pulling of cholesterol likely turns some HDL molecules into LDL molecules, since removing cholesterol from an HDL molecule results in a lower density lipid molecule.) or possibly by its effects on the liver, affecting the composition of the liver's cholesterol output.