WASHINGTON — The spurious story line alarmed Americans. It spread like wildfire, distorting facts into outlandish fictions, despite being attributed only to obscure sources. And it inflamed already divisive politics before the United States government concluded — perhaps too late — that it was a Russian disinformation campaign.

“We would take any misinformation like that very seriously,” Sarah Huckabee Sanders, the White House press secretary, told reporters at a briefing on Monday.

Ms. Sanders was responding to a question about how the Trump administration would view a “disinformation campaign by a foreign government” — specifically, fabricated and sensational news distributed by Moscow in the 2016 election. But presidents have for decades dealt with the phenomena of Russian-peddled fake news, if under a different name.

It is rooted in a Cold War plot that fanned a conspiracy theory about the origins of AIDS, earning a rebuke from the Reagan administration, and now serves as a lesson about Russian disinformation and the subversion of weighty issues worldwide.