Skipper Ben Stokes (31) is among several England players who skipped Wednesday's optional training session after being laid low by the bug.
Former captain Joe Root (31) was one of five players who did attend, and he later spoke at the pre-match news conference.
"As far as I'm aware, a few guys are not feeling 100%," Root said. "I didn't feel great yesterday and I woke up a lot better today. So hopefully it's a 24-hour virus. Don't think it's like food poisoning or COVID or anything like that.
"We are trying to do absolutely everything right to make sure we are prepared for this game."
Root effectively ruled out leading England again should Stokes miss the match with illness.
"I don't think so... Hopefully that's not even a conversation that we need to entertain," he added.
For their time in the country, England have hired a chef after receiving feedback from players who visited Pakistan between September and October for a seven-match T20 series.
The Pakistan Cricket Board (PCB) said it was in touch with its English counterpart and added late on Wednesday that the two unanimously agreed to delay the decision on the commencement of the test until 7.30 am local time (0230 GMT) on Thursday.
"The two boards also agreed, subject to the England players not recovering well enough to take the field on Thursday, then the test will commence on Friday and will be a five-day match," the PCB said in a statement.
"In this scenario, the schedule of the second test in Multan and the third test in Karachi will remain unaffected and will be played as per the original schedule - December 9-13 and December 17-21."
Pakistan captain Babar Azam wished the English players a speedy recovery.
"I hope they get better soon. We'd like to face a full-strength England tomorrow."
Bazball Away Test
Coach Brendon McCullum and captain Ben Stokes have championed England's aggressive, high-risk high-reward style and it is unlikely they will take a more conservative approach in the three-test tour, which starts on Thursday.
Veteran pace bowler James Anderson said England's only thought was winning.
"We've got a captain and coach that don't want draws. We're not playing for draws," Anderson told BBC Sport.
Six wins in their last seven matches at home vindicate England's new approach but fans are keen to see if it can work just as well abroad.
The tourists have made a bold decision by selecting batting all-rounder Livingstone, a highly sought-after Twenty20 player, to make his test debut despite not having played red-ball cricket this year.
Anderson was part of the England team which toured Pakistan in 2005, though he played only the one-day leg of that series.
Security concerns kept England away from Pakistan for the next 17 years until they arrived in September to play seven T20s on the first leg of their tour.
Anderson felt his experience from that 2005 tour would count for little and said England would have to adapt quickly to conditions in Rawalpindi, Multan and Karachi.
"Seventeen years is a long time," the 40-year-old said.
"It's a completely different team we're playing against, completely different conditions.
"It's about adapting when we get out there. We talk a little bit about the opposition but mainly concentrate on what we do well - that's what we did in the summer."
Ben Duckett returns to the England line-up for the first time since 2016, and will open the batting with Zak Crawley, while Joe Root is expected to share Jack Leach's spin burden along with Livingstone.
Hosts Pakistan will hope 19-year-old Naseem Shah can soften the blow of pace spearhead Shaheen Afridi's absence.
"When your best bowler gets injured, it makes a big difference," Naseem told reporters.
"So we have to take that responsibility and make good use of the new ball."
