Final answer:
The single most important host factor influencing the occurrence of UTIs is female sex, mainly due to anatomical differences and proximity of the urinary tract to sources of bacteria.
Step-by-step explanation:
The single most important HOST factor influencing the occurrence of Urinary Tract Infections (UTIs) is female sex. This is due to the anatomical differences between men and women, specifically the shorter urethra in females which allows for bacteria, often Escherichia coli, to enter the urinary tract more easily from the anus. Frequent sexual intercourse, certain types of contraception like spermicides or diaphragms, hormonal changes, and conditions that impair bladder emptying such as an enlarged prostate or catheter use, also contribute to the risk of developing UTIs. Good hygiene is crucial for reducing the risk of UTIs that result from fecal contamination.