According to  Professor Mark Wilcox, Clinical Director of Microbiology at Leeds Teaching Hospitals NHS Trust, whether you close the toilet lid before you flush, could have an impact on the spread of infections.

(Seemingly leaving the lid up when you flush can allow a cloud of bacteria to explode into the air, settling on nearby surfaces. I won’t go into the specifics, but they tested the hospital superbug C. difficile, and found that when flushing, bacteria was transported up to 10 inches above the toilet seat when it was open, and was found on the cistern, to the right and left of the toilet seat, and on the floor, in fact it was still detected in the air up to 90 minutes later, but when the lid was closed no C. diff. was recovered on any surface.  So, men, now you know why us women get so annoyed when you leave the lid up.)