The US has lifted a 22-year immigration ban which has stopped anyone with HIV/Aids from entering the country.
President Obama said the ban was not compatible with US plans to be a leader in the fight against the disease.
The new rules come into force on Monday and the US plans to host a bi-annual global HIV/Aids summit for the first time in 2012.
The ban was imposed at the height of a global panic about the disease at the end of the 1980s.
It put the US in a group of just 12 countries, also including Libya and Saudi Arabia, that excluded anyone suffering from HIV/Aids.
The BBC's Charles Scanlon, in Miami, says that improving treatments and evolving public perceptions have helped to bring about the change.
Rachel Tiven, head of the campaign group Immigration Equality, told the BBC that the step was long overdue.
"The 2012 World Aids Conference, due to be held in the United States, was in jeopardy as a result of the restrictions. It's now likely to go ahead as planned," she said.
In October, President Obama said the entry ban had been "rooted in fear rather than fact".
He said: "We lead the world when it comes to helping stem the Aids pandemic - yet we are one of only a dozen countries that still bar people with HIV from entering our own country."
Read it at the BBC.