Diabetes, skin cancer and syphilis have something unexpected in common: they can cause a sensation of bugs crawling on or under the skin, known as formication.
It can be described as tingling, burning, stinging and itchy.
However, patients who experience formication typically do not have a fixed belief that they have been infested by parasites, and can accept proof showing otherwise.
This differs from delusional parasitosis, where the patient is immovably convinced that insects have invaded their bodies.
The intense belief can disrupt the work and social life of patients. This is because they are "constantly preoccupied with finding ways to remove the perceived infestations from their bodies", said Dr Tay Liang Kiat, a consultant dermatologist at the Dermatology and Surgery Clinic, a private practice.
A retiree in his 60s who wanted to be known only as Mr Lee would spend three hours bathing and checking his body for insects daily.
The delusion started three years ago after he went on an overseas trip and felt that his hotel room was not clean - particularly the mattress.
At the time, he believed that insects and mites had crawled on his scalp and skin, sometimes entering his eyes, nostrils, ears and mouth, and laid their eggs under his skin.
Mr Lee also claimed he could see bugs falling from his body and onto the floor when he bathed. And when he saw people near him scratching themselves, he thought he had spread the bugs to them.
Said Dr Chan Keen Loong, a senior consultant in psychiatric medicine at Khoo Teck Puat Hospital: "As a result, he seldom went out. He is frustrated that his family cannot see the insects." Dr Chan added that Mr Lee is still receiving treatment.
In general, the injured skin is treated with topical creams while oral medication, such as anti-psychotics, may be prescribed.
Some patients resort to extreme measures to stamp out the imaginary insects, noted Dr Tay.
"There are severe cases where patients use tools like knives in their attempts to dig out these parasites, and use all sorts of disinfectants and insect repellents."
Overseas reports have described sufferers jamming burning cigarettes into the ear to "fumigate" the bugs inside, as well as taking a bath in insecticide-laced water.
Patients also fear spreading the insects to their loved ones.
"One grandmother had not held her grandchild since he was born four years ago," said Dr Chan.
Many sufferers will doctor-hop, seeking treatment at different places to quell the non-existent infestation. They often end up thinking the doctors are incompetent, said Dr Chan.