What pneumonic plague is
Pneumonic plague is an infection of the lungs by Yersinia pestis, the bacterium behind every form of plague. It is primary when someone inhales the bacteria, from another patient or an infected animal, or secondary when untreated bubonic or septicemic plague spreads to the lungs.
It is rare. Bubonic plague makes up roughly 80 to 95 percent of cases worldwide, according to WHO. Of 913 US cases between 1900 and 2012 with a known form, 74 (8 percent) were pneumonic. But pneumonic plague has the highest fatality of all forms and is the reason plague remains a disease that public-health systems treat as an emergency.
How it spreads
Pneumonic plague is the only form that spreads between people. A patient coughs out large respiratory droplets that fall within a short distance. CDC puts the risk zone at about 6 feet (2 metres) and notes that transmission "has been reported most commonly among an infected patient's caregivers or others living together".
- There is no evidence of airborne spread over distances, as with measles. Droplets do not stay suspended for long.
- Patients are most infectious in the last hours of illness, when coughing and sputum peak.
- Infected animals can also cause it: in the US, rare pneumonic cases follow exposure to sick cats.
- Person-to-person spread has not been documented in the US since the 1924 Los Angeles epidemic.
Symptoms and incubation
Symptoms appear fast. WHO says incubation "can be as short as 24 hours"; CDC and ECDC give 1 to 3 days after inhalation. The illness starts like a severe flu and then turns into a rapidly worsening pneumonia:
- sudden high fever, headache and weakness
- shortness of breath and chest pain
- cough, often with bloody or watery sputum
- without treatment, respiratory failure, sepsis and death
Early signs are not specific, which is why a recent possible exposure (a patient with plague, a sick animal, a laboratory accident, travel to an active focus) matters more for diagnosis than the symptoms alone. See all symptoms by form.
Pneumonic vs bubonic plague
| Pneumonic | Bubonic | |
|---|---|---|
| Infects | Lungs | Lymph nodes |
| Usual source | Droplets from a patient or animal; spread from other forms | Flea bite, handling infected animals |
| Incubation | From 24 hours; 1 to 3 days | 1 to 7 days (WHO); 2 to 8 days (CDC) |
| Person to person | Yes, close contact | Rare |
| Untreated | Almost always fatal | 30 to 60 percent fatal |
| Share of cases | Rare (8% in the US historically) | 80 to 95 percent worldwide |
Treatment
Pneumonic plague responds to common antibiotics. CDC lists ciprofloxacin, levofloxacin, moxifloxacin, gentamicin and streptomycin as first-line drugs for adults, for 10 to 14 days. WHO added the fluoroquinolones to its first-line list in 2021.
Timing decides the outcome. Patients "previously considered incurable" recovered when treated within the first 24 hours of symptoms, and CDC advises that antibiotics be given within 24 hours of onset. Doctors are told never to wait for laboratory confirmation. Natural antibiotic resistance in Y. pestis is rare. More on treatment and vaccines.
Contacts and prophylaxis
People who had close, unprotected contact with a pneumonic plague patient receive 7 days of preventive antibiotics, usually ciprofloxacin, levofloxacin, moxifloxacin or doxycycline. Health workers who used standard and droplet precautions do not need them. Patients stop being a droplet risk after about 48 hours of effective antibiotics with clinical improvement.
Because pneumonic plague can spread and kill quickly, countries must assess every event with WHO's IHR Annex 2 instrument, and serious or unusual events must be notified to WHO.
The 2026 Irkutsk case
In October 2026 a laboratory technician at the Irkutsk Anti-Plague Research Institute died after a severe pneumonia. Regional media and Telegram channels report pneumonic plague after a lab accident. Russia's health watchdog says the cause was "pneumonia of undetermined etiology" and found no plague among 197 contacts. We track every statement on the Irkutsk briefing.
Quick answers
Is pneumonic plague contagious?
Yes. It is the only form of plague that spreads from person to person, through large respiratory droplets during close contact, typically within about 2 metres. It does not spread through the air over distances like measles.
How fast does pneumonic plague kill?
Without treatment it can be fatal within 18 to 24 hours of the first symptoms and is almost always fatal. With antibiotics started within 24 hours of symptom onset, most patients recover.
What is the incubation period of pneumonic plague?
As short as 24 hours according to WHO; CDC and ECDC give 1 to 3 days after inhaling the bacteria.
Reference information, not medical advice. With symptoms after a possible exposure, call your local emergency number.