Hillary Clinton on Friday opened up about the challenges of the campaign trail, which she said can be "especially tricky for women."

During an interview on "The Tonight Show Starring Jimmy Fallon," Clinton -- who on Thursday made her first public appearance since it was announced she had been diagnosed with pneumonia -- was asked about how she balances the seriousness of the campaign with the need to remain positive and friendly with voters she meets on the trail.

The Democratic presidential nominee conceded it's not easy. "It is especially tricky for women," she said. "It is a constant balancing act -- how do you kind of keep the energy and the positive spirit while taking seriously what you need to."

Clinton referenced recent criticism she faced from the Republican National Committee for not smiling enough during a national security forum.

"The Republicans were saying, 'Oh, she looks so serious,'" she explained. "Well, you don’t talk about ISIS with a big grin on your face. They’re a barbaric, evil group that we have to defeat and wipe out."

Fallon also teased Clinton about her health.

As Clinton walked on stage, the host put on a white, medical mask and took out a bottle of antibacterial soap.

Clinton, who was diagnosed with pneumonia on Sunday, laughed and gave him a big high-five, seemingly entertained by the joke.