The team will have a lot to deal with on the next episode of NCIS, including a mystery man and a mystery illness.

The episode teaser shows sick children whose illness keeps progressing. While some of the team is losing hope, others are urging for positive attiudes.

"You know what's infectious during the holidays?" Abby Sciuto says. "Optimism is infectious. Faith is infectious."

Leon Vance, who has been dealing with the death of his wife, will have a mystery man appear at his door.

"You have no business coming here," Vance tells the man, who replies, "I'm not looking for trouble."

The gentleman is presumably the person the show's executive producer, Gary Glasberg, told TV Guide about in August.

"We'll be casting the father of Vance's wife," he said. "We think it will really have some resonance."

Vance's spouse was murdered in the Season 10 episode, "Shabbat Shalom," after she was hit by a machine gun fire. Following the hit, NCIS Medical Examiner Dr. Donald Mallard performed an autopsy to see what killed Jackie. He explained that she was shot in the temporal lobe of her brain.

The episode, titled "Homesick," airs on Dec. 17.