Sentence:  Of the three potential means (carriage on the body and legs, regurgitation and defecation) by which <WSD>houseflies</WSD> can transmit pathogens, one involves passage through the gut. During passage through the <WSD>housefly</WSD>, pathogens may replicate within the gut. 
Meaning of the word marked with <WSD> tags:  The common housefly, "Musca domestica", that frequents most homes and spreads some diseases.