Penicillin is a short story by the pseudonymous jespah.
On January 2, 2154, on the Enterprise (NX-01), Jay Hayes returns from a mission to a Xindi sphere with a cough. In competition with Malcolm Reed, he does not want anyone to know, but Lili O'Day hears him coughing, and offers to make him something to help. Swearing secrecy, she demands a small payment from him, that he smile on occasion.
At the end of his life, Hayes recalls this incident, and wills her his lucky nickel, in full and final payment for his chicken soup debt (In Between Days: Everybody Knows This is Nowhere).