The particular strain of the bug, which shut down Cargilfield Preparatory school and left two elderly victims fighting for their lives yesterday, can be found in compost. 
Four Scots gardeners have been struck down by deadly Legionnaires' disease - caught from compost.
Two elderly victims were fighting for their lives in intensive care, while two others have been treated in hospital.
A school was also closed down yesterday after a member of staff was diagnosed with the bug.
Health bosses issued a warning to all gardeners over the rare strain of the killer bug. They are being urged to wear facemasks when handling compost.
All four gardeners affected by the Legionella longbeachae strain come from the Lothian area and are aged between 62 and 84.
NHS Lothian consultant in public health Dr Richard Othieno said "This type of Legionella is quite rare in that, unlike other strains, it has never been identified in man-made water systems, such as cooling towers.
"We are working with experts to trace the source of the infection and samples of the compost have been  sent for testing"
"We know that each of the four cases are keen gardeners and had purchased different products containing compost to acquiring the infection".