Marathon swimmer Diana Nyad was forced to quit her 103-mile swim from Cuba to Florida on Sunday morning in her third attempt because of Portuguese man o’ war stings that could have killed her.

Diana Nyad shows Portuguese man o’ war stings Sunday. (Michael Marrero/AP)

The 62-year-old’s face and lips were left swollen from the jellyfish-like creature’s tentacles, and her arm was covered in a long, cross-shaped welt.

This Portuguese man o’ war is covered in oil after the Deepwater Horizon spill. (Carol Guzy/The Washington Post)

The Portuguese man o’ war is responsible each year for thousands of human stings, which cause severe pain and giant welts, and can lead to an allergic reaction, fever, shock, a disruption to heart and lung function, and even death.

The best treatment for man o’ war stings is to avoid further contact, as Nyad was suggested to do, as well as remove the creature’s remnants from the skin and apply salt water to the area.

The jelly-like creature is found in warm-water seas such as the Pacific, where it floats on the surface of the open ocean with its air bladder keeping it afloat. It is propelled only by winds and tides.