Being transgender is no longer classified as a mental illness by the World Health Organization – a key sign of progress for an often-marginalized community.

WHO announced Monday that in its newly released edition of the International Classification of Diseases, gender incongruence will now be classified as a sexual health condition.

Gender incongruence is "characterized by a marked and persistent incongruence between an individual’s experienced gender and the assigned sex," according to the International Classification of Diseases (ICD).

For example, a person who was born with a penis and is biologically male but identifies as female. Not every transgender person has gender incongruence, said Dr. Jennifer Conti, a fellow at Physicians for Reproductive Health.

"It was taken out from the mental health disorders because we had a better understanding that this wasn't actually a mental health condition and leaving it there was causing stigma," said Dr. Lale Say, coordinator of WHO's Adolescents and at-Risk Populations team. "So in order to reduce the stigma while also ensuring access to necessary health interventions, this was placed in a different chapter."

Why was being transgender originally classified as a mental illness?

LGBTQ identities have long been conflated with mental illness.

Homosexuality was originally classified as a mental disorder; in 1974, it was removed from the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorder, though a category remained for people who were in conflict with their sexuality. This was removed in 1987.

WHO didn't change its definition of homosexuality until 1992, when it included a note that "sexual orientation by itself is not to be classified as a disorder."

"We’ve historically misclassified a lot of conditions in medicine because of a combination of stigma, fear and misunderstanding," Conti said.

Why is being transgender now considered a sexual health condition?

While many believe gender incongruence shouldn't be classified in the ICD at all, others argue that it's easier for transgender people to seek hormonal or surgical treatment if gender incongruence is included, Conti said.

According to the WHO, "there remain significant health care needs that can best be met if the condition is coded under the ICD."

Transgender people often have trouble accessing health care because of poor knowledge among healthcare providers, discrimination, financial barriers and socioeconomic barriers. They're also at a higher risk of developing HIV and experiencing sexual violence.

How will this affect the transgender community?

For the transgender community, the WHO's decision is seen as a step toward acceptance in a society that often discriminates against it or fails to understand what being transgender means.

"By changing the class of this condition, the WHO is effectively saying to everyone and to the world that this is not a mental disorder and we support people who are transgender," Conti said. "It’s a really meaningful step because it promotes inclusivity, it promotes acceptance."

How are people reacting to the change?

On Twitter, many hailed the decision as a sign of progress.

Others were surprised that being transgender had been categorized as a mental illness for this long.