The Australian Sunday Telegraph reported over the weekend that the 32-year-old "Baywatch" star became sick with what's believed to have been typhoid or a similar bacterial infection while filming the show, which follows his travels "off the grid" in the jungle for 21 days "with nothing but basic gear, a guide partner and a will to survive," according to The Hollywood Reporter.
Zac's condition was so severe he was reportedly airlifted to Brisbane, Australia, then transported to a private hospital shortly before Christmas. He was able to be discharged a few days later and returned home to the U.S. on Christmas Eve, according to the Telegraph.
Typhoid fever is a potentially life-threatening gastrointestinal infection caused by Salmonella enterica typhi bacteria, which can be found in contaminated food and water, according to the nonprofit travelers' health awareness organization IAMAT.org.
The condition usually causes fatigue, fever, headache, lack of appetite, malaise and an enlarged liver as well as gastrointestinal problems. Severe cases can cause intestinal hemorrhage. The illness is usually treated with antibiotics. IAMAT recently warned there's been an increase in antibiotic resistance to the bacteria.
News of the actor's illness apparently had fans and others so concerned Zac hopped on Twitter and Instagram on Sunday, Dec. 29, after a month-long hiatus to clarify that he's now feeling much better.
"Very thankful to everyone who has reached out," he shared. "I did get sick in Papua New Guinea but I bounced back quick and finished an amazing 3 weeks in P.N.G. I'm home for the holidays with my friends and family. Thanks for all the love and concern, see you in 2020!"
As for his show, "Killing Zac Efron" — which is due out on the new short-form mobile platform Quibi — Zac told THR the project appealed to him because he tends to "thrive under extreme circumstances and seek out opportunities that challenge me on every level."
Zac is also listed as an executive producer for the show via his company, Ninjas Running Wild. The series is expected to air after Quibi's launch in April.