Top international golfer, Rory McIlroy, has pulled out of the Olympics in Brazil because of concerns over a virus called Zika.
McIlroy said that although the risk was low, his health and that of his family came first.
He had been due to compete for Ireland at the Rio de Janeiro games.
Vijay Singh and Australia's Marc Leishman have already pulled out of the games because of Zika worries.
But the International Olympic Committee, following World Health Organisation advice, said it has "total confidence" the Games will be safe for athletes.
What is the Zika virus?
The Zika virus is a disease which is spread by a type of mosquito, called the Aedes mosquito, found in some hot countries like Brasil.
Most people who catch Zika aren't affected very badly but it is much more serious for women who are pregnant.
Scientists say they think it may affect how a baby grows before it's born.
Other people might feel unwell, as if they have flu, but it doesn't usually last more than a week.