Larry Kramer, the playwright, novelist, and activist who was one of the first to lead the fight against the AIDS crisis, has died at the age of 84, the New York Times reports.

Kramer, who was diagnosed with HIV in 1988, died of pneumonia, his husband said.

Kramer started his career as a screenwriter, earning an Oscar nomination for the 1969 film Women in Love. He transitioned to playwriting, and published his first novel, the controversial satire Faggots, in 1978.

In 1982, he co-founded Gay Men’s Health Crisis, a service organization that aimed to help people suffering from AIDS, at the time a mysterious disease that seemed to disproportionately affect gay and bisexual men.

Three years later, his play The Normal Heart, now considered one of the best of the 20th century, debuted in New York. The semi-autobiographical play follows a writer and activist whose partner is dying of AIDS.

Kramer is also considered the godfather of ACT UP, the influential direct-action group known for its bold activism. The group is credited with raising awareness of AIDS throughout the country.

Kramer’s most recent book, The American People, Volume 2: The Brutality of Fact, was published in January. A reviewer for Kirkus called the novel “idiosyncratic, controversial, and eminently readable: a masterwork of alternative history.” The book was a sequel to The American People, Volume 1: Search for My Heart, published in 2015. The two novels form Kramer’s vast, imaginative account of AIDS as it intersects with American history.

On Twitter, Kramer’s admirers paid tribute to him. “When Americans tried to ignore the AIDS crisis, Larry Kramer and his allies in ACT UP forced the nation to reckon with the public health emergency head-on,” author Kevin M. Kruse tweeted. “His work remains a model for aggressive activism in the face of a pandemic.”

And journalist Dan Savage wrote, “Larry Kramer valued every gay life at a time when so many gay men had been rendered incapable of valuing our own lives. He ordered us to love ourselves and each other and to fight for our lives. He was a hero.”

Michael Schaub is a Texas-based journalist and regular contributor to NPR.