Six months after saying he would step down as CEO of Biogen Inc., George Scangos has landed at a new California biotech startup backed by the Bill & Melinda Gates Foundation.

Vir Biotechnology announced its launch Friday and said Scangos had been named chief executive. The San Francisco company said it plans to take advantage of “breakthroughs in immune programming’’ to develop treatments for infectious diseases.

“The opportunity to lead Vir is one I could not pass up. There is a tremendous global need for effective therapies and preventions for infectious diseases of considerable public health importance,’’ Scangos said in a statement. “Success would mean alleviation of a lot of human suffering as well as meaningful financial returns for Vir investors.’’

Another Biogen alum, Vicki Sato, who worked at the company as a vice president and was later president of Vertex Pharmaceuticals Inc., will chair Vir’s board.

According to the announcement, Vir is backed by the Gates Foundation and Chicago-based ARCH Venture Partners. Vir did not say how much it has raised in total but said ARCH has committed to investing at least $150 million.

Scangos, 68, said in July that he would leave Cambridge-based Biogen after six years at the helm. He said at the time that he planned to move to the San Francisco area. During his tenure, Biogen saw its revenue more than double.

But he stepped down as the company faced slowing sales growth for its multiple sclerosis drugs while working to develop risky but potentially transformative new drugs. Those drugs include a possible treatment for Alzheimer’s and a recently approved, pricey treatment for a rare genetic disease in children.

Biogen named Michel Vounatsos as its new chief executive last month.

Adam Vaccaro can be reached at adam.vaccaro@globe.com. Follow him on Twitter at @adamtvaccaro.