Former Masters champion Charl Schwartzel said three weeks ago that Zika was behind his decision to withdraw. Schwartzel said he and his wife plan to have more children and that he would play if he were single or if the Olympics were held elsewhere.
Marc Leishman of Australia also cited Zika because his wife's immune system has not fully recovered from nearly dying last year of toxic shock syndrome.
Others who withdrew " Adam Scott of Australia, Louis Oosthuizen of South Africa, Vijay Singh of Fiji and Graeme McDowell of Northern Ireland " did not specifically cite Zika. McDowell, who was next in line to replace McIlroy, said his wife is expecting their second child about two weeks after the Olympics and he had no plans to be out of the country in the weeks leading up to the birth.
Golf is returning to the Olympics for the first time since 1904. It already is set for the Tokyo Games in 2020, though the IOC will vote next year to determine whether the sport should be a permanent addition to the program.
Still to be determined is whether Jason Day, the No. 1 player in the world, decides to play.
Day began to hedge three weeks ago when he mentioned his wife wanted more children and said he was wary about going and needed to make a smart choice. Earlier this week at a preview day for the PGA Championship, he said he respected McIlroy's decision.
"It's a tough one going from trying to represent your country and trying to win a gold medal, but also understanding that it's a life decision that you have to make," he said.
Grace's decision is a blow to South Africa because its best three players " Grace at No. 11, Oosthuizen at No. 14 and Schwartzel at No. 23 " all have pulled out. Next in line would be Jaco Van Zyl and Brandon Stone, who won the South African Open at the start of the year but has yet to play in a major.
Grace said he wishes the South African teams well and apologized to fans for withdrawing.
"It would have been a huge honor to represent my country, so I really hope to be able to qualify again in four years' time," he said.