CDC Surveillance Classifications

Exposure Categories for Surveillance Purposes

“Confirmed” and “suspect” are CSTE surveillance case classifications, based on type of laboratory testing.

“Presumptive” and “possible” are healthcare exposure case classifications and can apply to both confirmed and suspect cases.
  • Travel: The patient spent at least one night away from home (in the state of residence, another state, or another country) in the 14 days before date of symptom onset, not including nights spent in a healthcare facility.
  • Presumptive healthcare: A case with ≥10 days of continuous stay at a healthcare facility during the 14 days before onset of symptoms.
  • Possible healthcare: A case that spent a portion of the 14 days before date of symptom onset in one or more healthcare facilities, but does not meet the criteria for presumptive healthcare-associated Legionnaires’ disease.
  • Assisted living: The patient spent a portion of the 14 days before date of symptom onset in a facility that provides custodial care without skilled nursing (e.g., assistance with activities of daily living, like bathing and dressing).
  • Senior living: The patient spent a portion of the 14 days before date of symptom onset in a facility that provides independent living for the elderly.

Note the following:

Patients can have both travel and healthcare exposures in the 14 days before date of symptom onset. The CDC Legionellosis Case Report Forms provide space to indicate multiple exposures.

A patient who spent ≥10 days of continuous stay in multiple healthcare facilities would be considered a presumptive healthcare-associated case for surveillance purposes.

If the date of symptom onset is difficult to define (e.g., in patients with chronic underlying illness), date determination is deferred to the judgment of the clinicians providing care and the public health officials performing the investigation.

Goals of Surveillance

  • Rapidly recognize cases that occur in similar locations or with similar exposures to detect outbreaks
  • Monitor and describe incidence and trends
  • Understand risk factors for infection
  • Identify opportunities for control and prevention
  • Monitor effectiveness of interventions implemented as part of an outbreak investigation

Public Health Impact

Outbreaks of Legionnaires’ disease can be difficult to detect because of:

  • Low attack rate
  • Time interval between exposure and symptom onset
  • Dispersal of people from the source of the outbreak
  • Underdiagnosis of cases

Strong surveillance helps to quickly identify new cases, epidemiologic links between cases, and the need for outbreak investigations. Such investigations are critical for detecting sources of transmission and implementing control measures. While state, territorial, and local public health officials are best positioned to systematically track Legionnaires’ disease cases and efficiently detect outbreaks within their respective jurisdictions, CDC is uniquely positioned to establish connections between cases that occur among residents of different jurisdictions. Most cases of Legionnaires’ disease are not associated with a known outbreak.1 However, improved ascertainment and reporting of exposure information helps identify possible sources of exposure and can increase detection of outbreaks.


  1. Hicks LA, Garrison LE, Nelson GE, Hampton LM. Legionellosis — United States, 2000–2009. MMWR Morb Mortal Wkly Rep. 2011;60(32):1083–6.