A fifth person diagnosed with legionella longbeachae is from Tayside and is being treated in Ninewells Hospital in Dundee.
Health Protection Scotland (HPS) confirmed the news last night after four people were diagnosed last Friday with the disease, which is being linked to gardening compost.
Two are still being treated in intensive care and the other two have been discharged.
The latest case is reported as making “steady progress” according to NHS Tayside.
A spokesman for the health authority added that staff are “encouraged” by the patient’s condition.
Legionella longbeachae is a rare strain of the disease and not contagious but is contracted from commercial growing material and other composted material like bark and sawdust.
Dr Martin Donaghy, the medical director of HPS, said: “Following the identification of five cases of an unusual form of legionnaires’ disease in Lothian and Tayside, Health Protection Scotland is coordinating an incident management team to investigate this issue.
“While the risk of becoming unwell from gardening activities (such as working with compost) remains very low, we would recommend good hygiene in relation to gardening wearing gloves, wearing a mask if dusty, particularly indoors, and washing hands immediately after use.”