Analysis: How experts track down the source of Legionella

If it is thought the cases are linked, a formal investigation will be carried out.

The first thing looked at is the epidemiological information. This is collected in a questionnaire about things like when they fell ill and where they were before falling ill.

Secondly, we look at the microbiology – specimens will be taken and tested in a hospital lab. If the germ is isolated, then the positive specimens are sent for further testing to the specialist laboratory.

The third type of investigation is environmental to define how people have been exposed to the germ and where this comes from (the source).

The germ lives in water. When contaminated water is released as a fine spray, it can be breathed in and cause illness.

If it appears the most likely source is outdoors, one of the first things we will look at are cooling towers. The legionella germ likes slime, a bit of rust and usually a bit of heat. It can grow well when a cooling tower is not properly maintained.

If the environmental, microbiological and epidemiological evidence all lines up, you have a very good idea about what has been the source.

• Martin Donaghy is medical director of Health Protection Scotland.