Final answer:
The likely etiology of the man's symptoms is Leptospirosis, a bacterial disease caused by Leptospira spirochetes, contracted from water associated with animal reservoirs like seals.
Step-by-step explanation:
The etiology of the symptoms described by the 20-year-old man, which include jaundice, muscle aches, and a generalized illness after swimming in water with seals, seems to be related to an infectious disease. Darkfield microscopy revealing spirochetes in his blood suggests he is likely suffering from Leptospirosis, a bacterial disease caused by bacteria of the genus Leptospira.
Leptospirosis can produce symptoms such as fever, headache, chills, vomiting, diarrhea, and rash with severe muscular pain. If the disease is not treated and progresses, it can lead to organ failure or meningitis due to the infection of organs such as the kidneys, meninges, or liver, a severe form known as Weil's disease. Jaundice is a common symptom when the liver is affected.