As oppressive summer heat continues into late August, air conditioning systems across South Korea are running non-stop. While cool indoor environments offer relief, poorly maintained cooling infrastructure can inadvertently foster infectious disease—most notably legionellosis, a bacterial infection transmitted through contaminated airborne water droplets.
Legionellosis encompasses two distinct clinical conditions caused by Legionella bacteria: Legionella pneumonia, a severe lung infection, and Pontiac fever, a milder flu-like illness. Managed as a Class 3 nationally notifiable infectious disease in South Korea, reported cases have risen sharply. According to Korea Disease Control and Prevention Agency (KDCA) statistics, South Korea recorded 640 legionellosis cases in 2025, a 41.6% increase from the previous year, with infections heavily concentrated during the summer months.
Transmission Mechanisms and Symptom Progression
Legionella bacteria naturally inhabit freshwater environments such as lakes, rivers, and thermal springs, multiplying most actively in warm water between 25°C and 45°C (77°F to 113°F). Human infection occurs when individuals inhale fine aerosols or micro-droplets dispersed from contaminated water sources.
Symptoms vary significantly depending on the form of infection:
Pontiac Fever: Typically develops after an incubation period of 24 to 48 hours, presenting with chills, fever, muscle aches, dry cough, sore throat, and nasal congestion. Because these symptoms mirror a standard summer cold, many patients overlook the illness. Most individuals recover spontaneously within two to five days without medical intervention.
Legionella Pneumonia: Far more severe, with an incubation period of up to 10 days. Patients often experience a sudden high fever approaching 40°C (104°F), accompanied by severe headaches, muscle pain, generalized weakness, and a persistent cough. As the infection progresses, patients may develop chest pain, shortness of breath, gastrointestinal distress such as diarrhea, or altered mental status.
High-Risk Populations, Diagnosis, and Treatment
While anyone can contract Legionella pneumonia, individuals aged 50 and older face significantly higher risks. Vulnerable populations also include smokers, heavy drinkers, individuals with chronic lung conditions, diabetic patients, and immunocompromised individuals. Left untreated, the infection can escalate to life-threatening complications, including lung abscesses, respiratory failure, septic shock, and acute kidney injury.
Because Legionella pneumonia cannot be reliably distinguished from other forms of bacterial pneumonia based on symptoms alone, prompt diagnostic testing is essential. Clinicians confirm the diagnosis through urine antigen tests, respiratory specimen cultures, and genetic testing (PCR), followed by targeted antibiotic therapy. Recovery generally occurs within two weeks of appropriate treatment.
Misconceptions About Household Air Conditioners
Contrary to common belief, standard household and vehicle air conditioners pose minimal risk of transmitting Legionella. According to Prof. Yoon Hee-young of the Department of Pulmonary and Critical Care Medicine at Soonchunhyang University Seoul Hospital, residential units do not circulate standing water internally in a way that generates bacterial aerosols, relying instead on closed-loop refrigerant systems with separate outdoor heat exchangers.
Instead, the primary vector lies in large commercial buildings and public facilities that utilize water-cooled systems. Industrial cooling towers, central hot-water networks, public bathhouses, decorative fountains, and water parks represent high-risk environments if water sanitation protocols are neglected.
"If a patient develops a sudden high fever, severe cough, or shortness of breath during the summer—or fails to improve under standard empirical pneumonia treatment—clinicians should strongly suspect Legionella pneumonia," said Prof. Yoon Hee-young. "Extra vigilance is essential for adults over 50, smokers, and individuals with underlying conditions such as chronic lung disease or diabetes."
