In 1937, a man employed to lay water pipes was found to be the source of a severe epidemic of typhoid fever. The man, an asymptomatic carrier of Salmonella enterica serotype Typhi, the bacterium that causes typhoid, habitually urinated at his job site. In the process, he contaminated the town's water supply with bacteria from his bladder. Over 300 cases of typhoid fever developed, and 43 people died before the man was identified as the carrier. How was this carrier identified?
a) nasal swab
b) sputum sample
c) throat swab
d) urine culture