Five things to know about blue-green algae. (Yeah, it’s bad. And it’s getting worse.)

Three years ago, a foul-smelling blue-green algae bloom so thick it looked like guacamole shut down the beaches of Martin County over the Fourth of July. Last year, blue-green algae blooms again popped up in Lake Okeechobee and the rivers connected to it. This year, there are small blue-green algae blooms appearing in the waters of Pinellas, Manatee and Sarasota counties.

It could be worse. A blue-green algae bloom in the northern Gulf of Mexico spurred Mississippi authorities to close all 21 of their beaches over the weekend. In 2014, a blue-green algae bloom in Lake Erie meant 500,000 people were without a source of drinking water.

What is blue-green algae, why does it keep appearing and what does that mean for your health? Here are some answers.

1. It’s not really an algae. Unlike Red Tide algae, blue-green algae are actually microscopic cyanobacteria that contain chlorophyll similar to true algae. They reproduce rapidly, just like algae when it blooms. They are typically found at or near the surface of the water, just like algae blooms. And they are known to produce toxins, just like what scientists call “harmful algae blooms.” They are believed to represent the earliest known form of life on the earth.

2. It can show up in either fresh or salt water. The blue-green algae bloom shutting down Mississippi’s beaches is growing in saltwater. The blue-green algae that has plagued both Lake Okeechobee and the Great Lakes thrives in freshwater. Ironically, a study by the U.S. Geological Survey last fall found that when the Lake Okeechobee algae traveled through the St. Lucie River to the Atlantic Ocean, the saltwater damaged its cell walls, releasing its toxins.

3. Those toxins are bad for humans. The Florida Department of Health says keep away from the algae blooms: “In high amounts, cyanobacteria toxins can affect the liver, nervous system and skin. Abdominal cramps, nausea, diarrhea, and vomiting may occur if untreated water is swallowed. Some people who are sensitive to the algae may develop a rash or respiratory irritation. If you come into contact with an algae bloom, wash with soap and water right away. If you experience an illness, please contact your healthcare provider.” There could be long-term effects of exposure, too. Earlier this year, University of Miami scientists found that dolphins that had come into contact with blue-green algae toxins displayed degenerative brain damage similar to Alzheimer’s, Lou Gehrig’s disease and Parkinson’s in humans.

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4. These blooms are not new, but their frequency is. The first freshwater blue-green algae bloom in Lake Okeechobee was reported in 1986, when it suddenly covered more than 100 square miles of the lake’s surface, leading to a New York Times headline that said, “Florida fears Lake Okeechobee is dying.” Lake Erie started experiencing regular toxic algae blooms in the 1990s, and they’ve continued to worsen. The 2016 bloom in Lake Okeechobee that spread to both of the state’s coasts was so bad that it became a political issue in the U.S. Senate campaign, and it was followed by another bloom last year, and the lake has already shown signs of a new bloom this year. In 2014, the Third National Climate Assessment predicted that climate change would lead to an increase in toxic algae blooms.

5. We can control the fuel for the blooms, if not what stops them. Nutrient pollution from agricultural and urban runoff, as well as from leaky septic tanks and sewer lines, causes the majority of freshwater cyanobacteria blooms, according to the Florida Fish and Wildlife Conservation Commission. Other conditions that contribute to blooms are stagnant water, resulting from a lack of natural flushing, and land clearing, which allows more polluted runoff to wash into waterways. Changing climate resulting in warmer ocean temperatures also is a factor for saltwater blooms, according to the National Oceanic and Atmospheric Administration. If the conditions that spur cyanobacteria growth change — for instance the water cools off or the flow of nutrients stops — then the bloom can fade.