Pressley's hair loss was caused by the autoimmune condition alopecia, and in response to her disclosure, women posted stories about their own alopecia diagnoses and photos of their hair loss, thanking Pressley for her honesty and authenticity in going public.
"As a Black woman, the personal is political," Pressley wrote on Twitter. "My hair story is no exception. Sharing a very personal story today to create space for others."
Pressley's disclosure came after longtime talk show host and actress Ricki Lake revealed her own decades-long struggle with hair loss on Instagram and later spoke to "Good Morning America."
In the process, Pressley and Lake joined the ranks of what's become a hallmark of celebrity behavior over the last decade or so. In the age of social media, a number of celebrities have become radically transparent about their medical conditions, starting public conversations about conditions that may have been previously been considered shameful and kept private.
Avril Lavigne and Justin Bieber have been candid about how difficult it is to have Lyme disease. Kim Kardashian has shared photos of her psoriasis flareups on Instagram. And Selena Gomez has been open about having lupus, even posting post-surgery photos following a kidney transplant she had in 2017.
It's a remarkable change, says Katrina Gay, who directs strategic partnerships at the National Alliance on Mental Illness.
It wasn't that long ago, as Gay tells it, that people would ask to have their NAMI magazines mailed to them in brown paper wrappers.
"They didn't even want their mailman to know [they were getting a magazine about mental health]," she said.
All that changed, she said, when Robin Williams died by suicide in 2014.
"There was a total turning point," Gay explained. While previously, many actors worried about talking about their depression or other mental problems publicly for fear of scaring off producers, or being overly identified by their condition, Williams' death shifted something in the culture.
People from different generations related to Williams and saw themselves in him, said Gay, whose phone rang off the hook with people wanting to share their stories in the aftermath of his death.
"I can no longer be silent," she recalls people telling her. "I have a story to tell."
Personal stories from people struggling with mental health problems can have a profound impact on stigma, according to Gay, and those narratives are even more effective when people relate to the individual telling the story.
It's part of why Williams' story was crucial. People deeply identified with him.
Personal stories change the way we see ourselves and the way we see others, Gay explained. "That's why the celebrity stories are so important. So many people relate to them."
In addition to driving down stigma, celebrity health campaigns can have a measurable effect on public health.
When Katie Couric got a colonoscopy on live television after her first husband died of colon cancer, it prompted a significant uptick in the colonoscopy screening rate in the nine months following her segment, according to a study published in JAMA Internal Medicine.
Sexually transmitted infections may be the exception to the rule
While the cultural effect of celebrities opening up about their medical conditions has encouraged conversation and humanized previously stigmatized conditions, one area where candidness seemed to backfire was when celebrities disclosed that they had sexually transmitted diseases.
"Sexually transmitted infections are so extensively stigmatized," said Janelle Marie Pierce, executive director of the STI Project, a group that she founded in response to stigma against herpes.
"Not even lay folk talk about it," Pierce added.
The resulting media blitz did little to help ease stigma against the condition, Pierce noted.
Unlike the media's portrayal of celebrities discussing their depression or alopecia, news articles about Sheen's disclosure tended to focus on whether his behavior -- like using drugs and having unprotected sex -- led to his diagnosis.
"Sharing his status actually contributed to existing stigma, because it reinforced the narrative [that people get HIV from living risky lifestyles]," Pierce said. "If those narratives are then highlighted by the media, further harm is done."
In contrast to a story like Williams', in which so many people related to the actor's experience with depression, the secrecy surrounding STIs may lead people to believe they don't know anyone who has a sexually transmitted disease.
"In reality they have probably known many people with STIs," Pierce said. "It's so much easier to apply stereotypes when you don't know someone personally."
For now, Pierce is still fighting against stigma and encouraging the media to avoid moralizing their coverage of sexually transmitted diseases.
"There have been a lot of missteps," she said. "But my overall opinion is that anytime we're talking about it, it's a good thing."
If you are in crisis, please call the National Suicide Prevention Lifeline at 1-800-273-TALK (8255) or contact the Crisis Text Line by texting TALK to 741-741.