An outbreak is defined as two or more cases where the onset of illness is closely linked in time (weeks rather than months) and where there is epidemiological evidence of a common source of infection, with or without microbiological evidence.
If a water system is implicated in an outbreak of Legionnaires' disease, emergency treatment of that system should be carried out as soon as possible.

HSG 274 Part 2 Appendix 2.3

In England and Wales, Legionnaires' disease is notifiable under the Health Protection (Notification) Regulations 201037 and in Scotland under the Public Health (Notification of Infectious Diseases) (Scotland) Regulations 1988.38

Under these Regulations, human diagnostic laboratories must notify Public Health England (PHE), Public Health Wales (PHW) or Health Protection Scotland (HPS) (see ‘Further sources of advice’) of microbiologically confirmed cases of Legionnaires' disease.

On average, there are approximately 300-350 reported confirmed cases of Legionnaires’ disease each year in England and Wales and it is thought that the total number of cases may be underestimated. About half of the cases are associated with travel abroad. Some clusters of cases and outbreaks occur for which no source of infection is confirmed. Legionnaires’ disease is notifiable under public health legislation and registered medical practitioners have a duty to notify the relevant public health agency when they suspect a patient has contracted the disease.

SMS Environmental is on hand at any time if you suspect you may have a Legionella problem and can offer expert advice and services to prevent an outbreak of Legionnaires' disease.