A<unk>  pneumophila bacterium (green) caught by a Vermamoeba vermiformis amoeba (orange) In the natural environment,<unk>  lives within amoebae such as Acanthamoeba spp., Naegleria spp., Vermamoeba vermiformis, or other protozoa such as Tetrahymena pyriformis. Upon inhalation, the bacteria can infect alveolar macrophages, where they can replicate. This results in Legionnaires' disease and the less severe illness Pontiac fever.<unk>  transmission is via inhalation of water droplets from a contaminated source that has allowed the organism to grow and spread (e.g., cooling towers). Transmission also occurs less commonly via aspiration of drinking water from an infected source. Person-to-person transmission has not been demonstrated; though, it could be possible in rare cases. Once inside a host, the incubation period may be up to two weeks. Prodromal symptoms are flu-like, including fever, chills, and dry cough. Advanced stages of the disease cause problems with the gastrointestinal tract and the nervous system and lead to diarrhea and nausea. Other advanced symptoms of pneumonia may also present. However, the disease is generally not a threat to most healthy individuals, and tends to lead to severe symptoms more often in immunocompromised hosts and the elderly. Consequently, the water systems of hospitals and nursing homes should be periodically monitored. The Texas Department of State Health Services provides recommendations for hospitals to detect and prevent the spread of hospital-acquired disease due to<unk>  infection. According to Infection Control and Hospital Epidemiology, hospital-acquired<unk>  pneumonia has a fatality rate of 28%, and the source is the water distribution system.<unk>  species typically exist in nature at low concentrations, in groundwater, lakes, and streams. They reproduce after entering man-made equipment, given the right environmental conditions. In the United States, the disease affects between 8,000 and 18,000 individuals a year. Sources of<unk>  Documented sources include cooling towers, swimming pools (especially in Scandinavian countries), domestic water systems and showers, ice-making machines, refrigerated cabinets, whirlpool spas, hot springs, fountains, dental equipment, soil, automobile windshield washer fluid, industrial coolant, and waste water treatment plants. Airborne Transmission The largest and one of the most common source of Legionnaires' disease outbreaks are cooling towers (heat rejection equipment used in air conditioning and industrial cooling water systems) primarily because of the risk for widespread circulation. Many governmental agencies, cooling tower manufacturers, and industrial trade organisations have developed design and maintenance guidelines for controlling the growth and proliferation of<unk>  within cooling towers. Research in the Journal of Infectious Diseases (2006) provided evidence that L. pneumophila, the causative agent of Legionnaires' disease, can travel at least 6 km from its source by airborne spread. It was previously believed that transmission of the bacterium was restricted to much shorter distances. A team of French scientists reviewed the details of an epidemic of Legionnaires' disease that took place in Pas-de-Calais, northern France, in 2003–2004. Of 86 confirmed cases during the outbreak, 18 resulted in death. The source of infection was identified as a cooling tower in a petrochemical plant, and an analysis of those affected in the outbreak revealed that some infected people lived as far as 6–7 km from the plant. Because of these risks, a UK legal mandate requires owners to notify the local authorities of any cooling towers a company operates. Cooling Tower Notification Recreational exposure As outlined above, cooling towers are well established as sources of<unk>  that may have a large impact on community exposure to the bacterium and Legionnaires Disease epidemics. In addition to cooling towers, use of swimming pools, spa pools, and other recreational water bodies has also been shown to increase risk of exposure to<unk> , though this differs by species of<unk> . In a review of disease caused by recreational exposure to<unk> , most exposures occurred in spas or pools used by the public (hotels or recreational centers) or in natural settings (hot springs or thermal water). Hotels and other tourist destinations have contributed to<unk>  exposure. Relative danger at commonly used facilities with heating and cooling water systems depends on several factors, such as: the water source, how much<unk>  is present (if there is any), if/how the water system is treated, how people are interacting with this water, and other factors that make the water systems so dynamic. In addition to tourists and other recreators, gardeners may be at increased risk for exposure to<unk> . In some countries (like Australia),<unk>  lives in soil and compost. Warmer temperatures and increased rainfall in some regions of the world due to climate change may impact<unk>  in soil, gardeners seasonal exposure to contaminated soil, and complex water systems used by the public. Exposure related to natural disasters and climate change Not only are<unk>  spp. present in man-made water systems and infrastructure, but this bacteria also lives in natural bodies of water, such as lakes and rivers. Weather patterns and other environmental factors may increase risk of<unk>  outbreaks; a study in Minnesota, USA, using outbreak information from 2011 to 2018 showed precipitation as having the greatest effect of increasing risk of<unk>  exposure when taking into account other environmental factors (temperature, relative humidity, land use and age of infected person). Weather patterns heavily relate to the established infrastructure and water sources, especially in urban settings. In the US, most cases of<unk>  infection have occurred in the summertime, though they were likely more associated with rainfall and humidity than summer temperatures. Severe rain patterns can increase risk of water source contamination through flooding and unseasonable rains; therefore, natural disasters, especially those associated with climate change, may increase risk of exposure to<unk> . Vaccine research No vaccine is available for<unk> sis. Vaccination studies using heat-killed or acetone-killed cells have been carried out in guinea pigs, which were then given<unk>  intraperitoneally or by aerosol. Both vaccines were shown to give moderately high levels of protection. Protection was dose-dependent and correlated with antibody levels as measured by enzyme-linked immunosorbent assay to an outer membrane antigen and by indirect immunofluorescence to heat-killed cells. However, a licensed vaccine is most likely still many years away.
Legionella
Mycoplasma
Streptococcus
Corynebacterium
Candida 
Bacillus anthracis
Cryptococcus
Bartonella