It was his yellow, bloody eyes that gave his illness away. The previously healthy 18-year-old showed up at an emergency department in the Netherlands after two days of fever, vomiting, and diarrhea. His heart was beating rapidly and his abdomen was a bit tender.

The whites of his eyes were splotched with blood, a sign that blood vessels on the surface of his eyes had burst. Areas that weren't bloodied were a jaundice yellow. Lab tests would later indicate he had acute kidney injury as well as liver dysfunction. But an equally important clue as to what was causing his acute illness was the mention that three weeks prior he had fallen into a canal.

In all, it was a textbook case, according to a report published recently in the New England Journal of Medicine. The man had a rare but severe bout of leptospirosis, which is a bacterial infection marked by fever, jaundice, kidney failure, and hemorrhage. The source: a fall into a canal that was likely tainted with the urine of infected rodents.

Leptospirosis is caused by the spiral-shaped bacterium Leptospira interrogans, which quietly infects a range of animals, setting up shop in the kidneys. The bacteria can be found harmlessly infecting pigs, horses, cows, dogs, a range of wild animals, even frogs. But they're most dangerous to humans by way of the common brown rat, Rattus norvegicus.

The bacteria are shed in an infected animal's urine, and people can become infected through direct contact or through exposure to contaminated water and soil. The spiraling invaders make their way into humans via cuts and mucus membranes, such as the eyes. Once in, they proliferate in the bloodstream and can attack the liver and kidneys. Severe cases can progress to meningitis, organ failure, and acute respiratory distress.

L. interrogans is found worldwide but is mainly endemic to humid tropical and subtropical areas. The World Health Organization estimates there are more than 500,000 cases of leptospirosis each year. According to the US Centers for Disease Control and Prevention, about 10 percent of cases are severe, and about 5 to 15 percent of severe cases are fatal. Antibiotics, such as doxycycline or penicillin, are effective at clearing the infection.

In the young man's case, a week's stay in the hospital and intravenous antibiotics cleared things up. At a three-week follow-up, all his symptoms had resolved.