Anthony S. Fauci, the nation’s top infectious-diseases expert and the face of the U.S. response to the novel coronavirus pandemic, is facing growing threats to his personal safety, prompting the government to step up his security, according to people familiar with the matter.

The concerns include threats as well as unwelcome communications from fervent admirers, according to people with knowledge of deliberations inside the Department of Health and Human Services and the Department of Justice.

Fauci, 79, is the most outspoken member of the administration in favor of sweeping public health guidelines and is among the few officials willing to correct President Trump’s misstatements. Along with Deborah Birx, the coordinator for the White House’s task force, Fauci has encouraged the president to extend the timeline for social-distancing guidelines, presenting him with grim models about the possible toll of the pandemic.