Fox News senior medical analyst Dr. Marc Siegel joins 'America Reports' to unpack the causes of pneumonia, its symptoms and how serious it can become following the death of Hollywood legend Diane Keaton.

Al Pacino is paying tribute to Diane Keaton, his "Godfather" co-star and former flame.

Keaton, who died on Oct. 11 after battling pneumonia, "was a wonder," Pacino described in a moving statement to Deadline.

"I am deeply saddened by Diane Keaton's passing," the "Serpico" actor began.

"When I first heard the news, I was shaken. Diane was my partner, my friend, someone who brought me happiness and on more than one occasion influenced the direction of my life. Though over thirty years has past since we were together, the memories remain vivid, and with her passing, they have returned with a force that is both painful and moving."

DIANE KEATON AND AL PACINO'S ON-OFF LOVE: THE TRUTH ABOUT HOLLYWOOD'S QUIETEST ROMANCE

Pacino and Keaton first met while filming the 1972 classic "The Godfather" - he starred as Michael Corleone, while she played the role of Kay, his wife.

They developed a romantic relationship that lasted for several years, though they never married.

TUBI STREAMS DIANE KEATON FILMS FOR FREE AFTER ACTRESS DIES AT 79: 'ANNIE HALL,' 'BABY BOOM' AMONG FEATURES

Pacino told Deadline, "She lived without limits, and everything she touched carried her unmistakable energy. She opened doors for others, inspired generations and embodied a once-in-a-lifetime gift that radiated through her work and her life."

He continued, "On screen, she was magnetic -- lightning and charm, hurricanes and tenderness. She was a wonder. Acting was her art, but it was only one of the many ways she expressed her imagination and creativity."

Pacino acknowledged that while people will miss Keaton, "more than that, they will remember her. She left a mark that cannot fade. She was unstoppable, resilient and above all, deeply human."

He closed his sentimental statement by writing, "I will always remember her. She could fly -- and in my heart, she always will."

While Keaton and Pacino met in 1971 when they began filming "The Godfather," their relationship reportedly didn't become romantic until after the movie wrapped filming. In recent years, the two have spoken out about the relationship.

LIKE WHAT YOU'RE READING? CLICK HERE FOR MORE ENTERTAINMENT NEWS

"I was mad for him. Charming, hilarious, a nonstop talker," Keaton told People in 2017. "There was an aspect of him that was like a lost orphan, like this kind of crazy idiot savant. And oh, gorgeous!"

She told the outlet at the time that she gave Pacino an ultimatum when he wouldn't commit to marriage. "I worked hard on that one. I went about it in not a perfect way," Keaton told People in 2017.

As she told The Sunday Times in 2017, "I didn't even want him to propose. I just thought maybe he would marry me, eventually. I thought, 'don't even propose -- let's just do it.' But that never happened, and that is a blessing for both of us. It would have been a nightmare for him."

She continued, "We're very eccentric, he needed a woman that was going to take care of him, I needed a man who would take care of me ... It was just very important that we left each other alone, said goodbye. But it wasn't my choice."

CLICK HERE TO SIGN UP FOR THE ENTERTAINMENT NEWSLETTER

Neither Keaton nor Pacino ever married, and although Keaton said their split wasn't her choice, Pacino clearly maintained fond feelings for his former co-star.

When Keaton was honored with an AFI Lifetime Achievement Award in 2017, he spoke during the ceremony, saying in part, "You're a great artist. I remember it, and I love it, and I love you forever."

CLICK HERE TO GET THE FOX NEWS APP

Fox News Digital's Janelle Ash contributed to this report

Diane Keaton's death leaves former partner Al Pacino deeply shaken: 'She was a wonder'