Incubation is the time between the moment the bacterium enters the body and the first symptom. For plague it is short.
- Bubonic plague: usually 2 to 6 days according to the World Health Organization and the European Centre for Disease Prevention and Control; the United States Centers for Disease Control and Prevention give 2 to 8 days.
- Pneumonic plague: 1 to 3 days, and it can be less than 24 hours.
- Septicaemic plague: not well defined; probably within days of exposure.
- Any form: the World Health Organization gives an overall range of 1 to 7 days. When doctors ask about exposure they count back 10 days, to be on the safe side.
In practice: if 10 days have passed since the last possible exposure and you are well, plague from that exposure is no longer expected.
How it begins — the same in every form. Suddenly. A person who was well in the morning has, by the evening, a high fever, chills, a severe headache, aching muscles and marked weakness. There may be nausea, vomiting, abdominal pain or diarrhoea. In the first hours it looks like a bad influenza, and that is exactly the difficulty.
What sets bubonic plague apart: the bubo. Together with the fever, or soon after it, one lymph node swells — most often in the groin, otherwise in the armpit or the neck, on the side of the body where the bacterium entered. It is:
- very painful — the World Health Organization describes it as excruciatingly tender;
- firm, with warm, reddened skin over it;
- usually single;
- fast: it appears and grows within hours to a day or two, not over weeks.
A bubo in the groin points to a bite on the leg. A bubo in the armpit points to the hand or arm — after handling an animal, for example. Sometimes there is a small blister or sore where the flea bit.
What sets pneumonic plague apart: the chest. Fever, headache and weakness come first. Then, within about a day, a cough, chest pain and growing shortness of breath. The sputum is watery at first, then streaked with blood. The person is worse by the hour.
What sets septicaemic plague apart. Often nothing local at all: fever, chills, extreme weakness, abdominal pain. Later, bleeding into the skin, and blackening of the fingers, the toes or the tip of the nose.
What it is mistaken for. Influenza. An ordinary infected lymph node after a wound or a scratch. Cat-scratch disease. Tularaemia. Appendicitis, when the swollen nodes are inside the abdomen. Any severe pneumonia. At the bedside a doctor cannot tell pneumonic plague from another severe pneumonia — which is why the history of exposure you bring with you matters as much as the symptoms do.
Source. WHO, Plague fact sheet, https://www.who.int/news-room/fact-sheets/detail/plague ; WHO, Manual for plague surveillance, diagnosis, prevention and control (2024), section 3, https://www.who.int/publications/i/item/9789240090422 ; United States Centers for Disease Control and Prevention, Signs and symptoms of plague, https://www.cdc.gov/plague/signs-symptoms/index.html ; European Centre for Disease Prevention and Control, Factsheet about plague, https://www.ecdc.europa.eu/en/plague/facts
What this is not. Reading, not medical advice. A list of signs is not a diagnosis; only a doctor and a laboratory can confirm or exclude plague.