Epidemiology and Outbreaks

Epidemiology is the study of the occurrence, transfer and control of disease.So epidemiologists monitor disease by measuring what they would expect to occur against what actually occurs. Eventually historical data will tell them to expect a ‘normal’ number of cases of disease in a given time period and location. If the number of cases of disease goes above this then it is an indication that something is going on – possibly an outbreak.

What is an Outbreak?

A working definition of an outbreak is ‘a higher than expected number of cases of disease in a location over a time period’. Surveillance of diseases uses the expected number of cases as a baseline to compare against the actual cases. This is used to work out the value of an intervention like a vaccination program, or to detect a disease outbreak. This can also spot differences between locations, seasons and communities.

In Australia the expected (background) number of cases of Legionnaire’s disease per annum is quoted per 100,000 of population. It is normal to report between 1.5 and 2.0 cases of disease each year.

Cooling Towers and Spas

It is normal practice for epidemiologists to plot an ‘epidemic curve’. This is a chart of the number of cases of disease reported over time. The resulting shape of the curve will give some idea about the nature of the outbreak. In the case of Legionnaire’s disease associated with a cooling tower outbreak the curve normally looks like the one below. Normally an outbreak lasts two incubation periods of the disease which is typically between 20-28 days. Why?

The source is a single cooling tower that emits a large burst of contaminated aerosol that may travel over a kilometre from the source. About half way through the first incubation period (around 5 days) cases that have been exposed to the aerosol cloud start to be reported. Of course there is a media frenzy.


By the end of the first incubation period most cooling tower operators have carried out a preventative disinfection of their cooling towers and the source of infection is over. Consequently this means the outbreak will subside and end within the next incubation period. No more infections occur. After an incubation period from the last reported case a cooling tower outbreak is usually deemed to be over.

Building Water Systems

Outbreaks from building water systems are more difficult to detect. In some way they don’t fit the definition of an outbreak as cases may turn up that are multiple incubation periods apart. As discussed in ‘Legionella species’ a much wider range of Legionella species may cause disease in building water systems.

Infection may be via aerosol inhalation or aspiration (sucking contaminated water into the lungs). The source of infection may be a single outlet in a building, or a room or a wing. The movement of aerosol and the amount generated is much less than cooling towers. The outlets may only produce enough aerosol to infect because it has been stagnant for a while and once occupied the room fittings are flushed by the occupant using them. Consequently much smaller numbers of individuals are exposed over much longer periods as a nett result. As a result there are less cases of disease over a much longer period of time. This makes surveillance more difficult. The building can only be implicated as a source of the cases if the infected person has been in the facility for at least one incubation period.

So the epidemic curve looks more like the one below.


Reported outbreaks of Legionnaire’s disease in hospitals often go undetected for years. This is because to recognise an elevation in the number of cases the data has have an annual review. So if single cases turn up irregularly they are easy to miss. For instance in one report disease cases continued unnoticed for more than 10 years.

This emphasises the need for clinical surveillance of disease. More importantly it emphasises that building water systems need a proactive water safety and risk management program. A program of routine water treatment will protect against disease transmission.

Further Reading

World Health Organisation (2007) Legionella and Prevention of Legionellosis

Department of Health, EnHealth (2015) Guidelines for Legionella control in the operation and maintenance of water distribution systems in health and aged care facilities.

 Legionella, water treatment, risk management plan, Legion