Legionella bacteria thrive in warm, damp environments and are commonly spread through air conditioning and water systems as well as potting mixes, mulches, composts and garden soils. This bacteria has the ability to cause serious illness if it is inhaled or ingested. It can be prevented through the use of some protective measures.
Always wear gloves.
Keep the mix damp when in use.
Avoid inhaling the mix.
Wash hand thoroughly after use.
Further information is available at the following Department of Health.