▲Venus Williams has helped revolutionize tennis, with a whopping seven Grand Slam Singles wins.

▲To many, Messi is the best soccer player in the world. However, you may not know about a serious health condition that could have stopped his career.

▲Messi was diagnosed with growth hormone deficiency when he was just a child. Barcelona F.C. paid for his treatment as a teen, and he has since earned the nickname "Little Man."

▲Muhammad Ali is considered one of the best athletes ever, but battled a serious disease until his late years.

▲In 1984, "The Greatest" was diagnosed with Parkinson's disease. In 1997, he set up the Muhammad Ali Parkinson Center, where the disease continues to be researched and treated.

▲Martina Navratilova is a tennis star, with a career spanning thirty years.

▲In April 2010, she went public about her breast cancer diagnosis. After surgery, the tumor was removed.

▲Wilma Rudolph was once the fastest woman in the world. At the 1960 games, she won three gold medals in track and field. Not bad, considering she had been paralyzed by the poliovirus as a child.

▲Of course, she recovered, but Rudolph's left foot had become deformed. By the time she was 12, she had already battled the measles, scarlet fever, and whooping cough. Still, that didn't stop her from becoming an Olympic champion.

▲Tim Howard has earned high praise as a goalkeeper. He has played for Premier League club Everton, as well as for the United States national team.

▲When he was nine, Howard was diagnosed with Tourette's, which is characterized by tics and involuntary actions. He is a patient-advocate for the disorder, and has opened up about his struggle in his memoir, 'The Keeper.'

▲Lou Gehrig is widely considered to be the first athlete who went public about their health condition.

▲"The Iron Horse" played for the New York Yankees from 1923 and 1939. His career was cut short due to amyotrophic lateral sclerosis, which became known worldwide. Some call ALS "Lou Gehrig's disease."

▲Mickelson, AKA "Lefty," is a pro golfer who has won three Masters titles. In 2010, he thought his career was all over after he started feeling pain in his joints.

▲The golfer was diagnosed with psoriatic arthritis. However, by treating the disease, Mickelson has still been able to compete at the highest level.

▲Austin rose to fame as a college basketball star, playing for Baylor University. He almost made it to the NBA in 2014, but was held back by a medical condition.

▲The player was diagnosed with Marfan syndrome the same year. After two years of treatment, he returned to playing basketball in 2016.

▲Eric Berry began his career with the NFL's Kansas City Chiefs as a defensive back in 2010.

▲In December 2014, Berry revealed that he had been diagnosed with Hodgkin's disease. Thanks to medical treatment and plenty of support, he made a full recovery in just nine months.

▲Many consider Kareem Abdul-Jabbar to be one of the best basketball players ever. In 2009, the NBA star announced that he was battling a deadly disease.

▲Abdul-Jabbar revealed that he had chronic myeloid leukemia in November 2009. He has since become an advocate to raise awareness of the disease.

▲Ashe broke boundaries by becoming the first black tennis player to win Wimbledon, the U.S. Open, and the Australian Open. However, his career was cut short by medical issues.

▲Ashe's career stalled in 1980, when he was diagnosed with a heart disease. Tragically, it didn't end there. The player announced that he had AIDS in 1992, and passed away just one year later. Despite not playing at a pro level until the end, he did a great deal of awareness and advocacy.

▲It's hard finding a figure skater who was as successful as Scott Hamilton. He won four consecutive U.S. and World championships, as well as an Olympic gold medal.

▲When he was a kid, Scott was diagnosed with a disorder that reduces the impact of nutrients. This gave him a small stature. Later in life, Hamilton was diagnosed with testicular cancer and brain cancer.

▲Pat Summitt is a special kind of sporting star. She started off playing basketball for the UT Martin Skyhawks and the United States national team, and went on to coach for the Tennessee Lady Volunteers team.

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Back in August 2011, Summitt announced that she had early-onset dementia. She retired from coaching one year later, and published a memoir about her struggles in 2013.

▲The South African runner was born with XY chromosomes, which gives her high levels of testosterone. She has battled opponents, as well as the International Association of Athletics Federations, who ultimately ruled that Semenya must undergo medical treatment to compete in future events. The ruling was widely criticized by many, including the World Medical Association.

▲In 2011, Williams revealed that she had been diagnosed with Sjögren's syndrome, which can cause extreme fatigue and muscle pains. Despite this, she went on to win Olympic gold medals and Wimbledon titles.

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As one of the most decorated track athletes, Joyner-Kersee took home three Olympic golds, as well as multiple wins at World Championships.

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Joyner-Kersee did all of that while suffering from asthma. She is one of the most famous athletes who overcame the inflammatory disease.

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Bronny James, the 19-year-old son of NBA star LeBron James, was hospitalized on July 24 after suffering cardiac arrest during basketball practice at the University of Southern California. The following day he was out of the intensive care unit and in stable condition.

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It's not easy being the best. Muhammad Ali, Lionel Messi, and Venus Williams are seen as some of greatest athletes ever. That said, did you know that they have all suffered from serious medical conditions? That's right, many sports stars have battled illnesses, diseases, and medical conditions in general.

Intrigued? Get to know these brave and bold athletes in this gallery.