The infection: Leptospirosis, caused by various members of Leptospira bacteria.
Symptoms and treatment: It can initially cause high fever, chills, muscle aches, and jaundice (yellowed skin and eyes). Sometimes, people can recover and later experience a worse and more life-threatening second phase of illness, one that can severely damage the liver, kidneys, and brain. The second phase is more common when the infection goes untreated with antibiotics.
Threat potential: The bacteria is spread through the infected urine of various animals, including rats, which can then end up contaminating nearby sources of water or soil for weeks to months. Though rare in the U.S., there have been recent spikes in places like New York and experts fear that a warmer climate will make cases more common.