Cleaver, who plays Eileen Grimshaw in the ITV soap, was taking part in a documentary called Corrie Goes To Kenya with fellow cast members Brooke Vincent (Sophie Webster), Ryan Thomas (Jason Grimshaw) and Ben Price (Nick Tilsley).
As part of the programme, the stars visited local tribes who are still struggling after last year's famine. They also aimed to raise awareness of HIV and AIDS.
Cleaver, 47, began to feel ill after returning to the UK. She was rushed to hospital and underwent emergency testing to rule out the possibility that she had contracted malaria.
After doctors determined that she was clear of the disease, the actress was sent home to recover, but continues to feel unwell. It is not known when she will be able to return to the soap.
"No-one really knows what's wrong with Sue, but she's been feeling very ill," a source told The Mirror.
"It started pretty much as soon as she got back from Kenya.
"Obviously serious alarm bells start ringing when you come back from Africa feeling unwell because you hear so many horror stories about people contracting diseases out there."
The source added: "Sue spent a lot of time with children in orphanages. She ate food cooked in the traditional way, often out in the bush. It means there were a great many times when she could have picked up a bug."
The paper reports that Cleaver was not due to film many scenes last week, meaning there was little disruption to the schedule.
"It may become a problem if she's off too much longer," the insider added. "Having said that, all the bosses are concerned about is her recovery."
Corrie Goes To Kenya is due to air later this month.