## Introduction

Legionnaires disease has a 25% mortality rate (Medscape)

## Hosts

Human and animal.

## Transmission / Exposure Route

Infection normally occurs after inhaling an aerosol (fine airborne particles) containing Legionella bacteria. [1]
Anywhere from 8,000 to 18,000 people are hospitalized each year in the U.S. with Legionnaires' disease. (EH&E) Between 1980 and 1998, 35% of Legionnaires Disease cases met definition of nosocomial infection.[2]
Disease is usually associated with man-made environments, such as cooling towers, whirlpools, and building water systems, where warm water (25C–42C) and biofilms support growth and survival of Legionella species. [3]

## Case Fatality Ratio

Legionnaires disease has a 25% mortality rate. (Medscape
Legionnaires' disease can be very serious and can cause death in up to 5% to 30% of cases. [5]
Between 1980 and 1998, the case fatality ratio decreased from 34% to 12% (P<0.001) for all cases, 46% to 14% (P<0.0001) for nosocomial cases, and 26% to 10% (p=0.05) for community-acquired cases. [2]

## Incubation Period

During an outbreak in the Netherlands the incubation period was reported to be 2-19 days (median 7 days, n=136), 16% of these individuals showed incubation times exceeding 10 days. [4]

## Burden of Disease

U.S. legionellosis cases reported annually increased 217%, from 1,110 in 2000 to 3,522 in 2009, and the crude national incidence rate increased 192%, from 0.39 per 100,000 persons in 2000 to 1.15 in 2009. [6]
Between 1980 and 1998, the median annual number of cases was 360 cases/year. [2]
Elderly individuals, smokers, and people with underlying respiratory or immunocompromising conditions are at increased risk for Legionnaires disease. [7]

Figure 1. No. of cases of legionnaires disease (LD) and rate per 100,000 US residents, by age group, 1980–1998.[2] (Surveillance data collected by the Center for Disease Control as presented in Benin, Andrea L, Robert F Benson, and Richard E Besser. “Trends in Legionnaires Disease, 1980–1998: Declining Mortality and New Patterns of Diagnosis.”)

#### Symptomology

Symptoms include a high fever, chills, cough, muscle or headaches. These symptoms are common to most forms of pneumonia so diagnosis relies on chest X-rays and tests performed on sputum, blood, and urine. [5]

## Microbiology

A thin, aerobic, pleomorphic, flagellated, non-spore forming, Gram-negative bacterium of the genus Legionella.[8] [9]
Legionella species are weakly gram-negative bacteria found primarily around fresh water environments, such as lakes and streams, where the bacteria use free-living amoeba as hosts for intracellular survival and multiplication.[3]

### Route: inhalation, Response: infection

#### exponential

$P(response)=1-exp(-k\times dose)$

k = 5.99E-02
ID50 = 1.16E+01

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