Leptospirosis

Leptospirosis is an infection caused by a bacterium called Leptospira. It's a zoonosis, meaning an infection that passes from animals to people, and it's the most common one in the world. Most people catch it from the urine of infected animals: sometimes by touching it directly, and sometimes because that urine ended up in water or soil.

Here's what happens inside the body. The bacterium gets in through skin that has a cut or scrape, or through the mucous membranes, the moist linings inside places like the mouth and nose. From there it moves through the lymphatic system, a network of tiny vessels that collects fluid from the tissues, until it reaches the blood. The bloodstream works like a road network that reaches everywhere, so the bacterium can travel all over the body, but it usually ends up mostly in the liver and kidneys.

Most often, the infection feels like the flu and goes away on its own. That's the anicteric form, which means without jaundice, the yellowing of the skin and eyes. The icteric form, also called Weil disease, is the severe one: jaundice shows up, along with kidney failure, when the kidneys stop filtering properly; hemorrhage, meaning bleeding; and respiratory distress, which means it's hard to breathe.