Plague DataUpdated October 16, 2026
Home / How we count

How we count

The rule behind everything here: every number comes from an official source, has a date, and links to where it came from.

Which sources can change a number

  • World Health Organization: its yearly plague reports (the Weekly Epidemiological Record) and Disease Outbreak News.
  • U.S. Centers for Disease Control and Prevention: yearly plague statistics and weekly notifiable disease tables.
  • National and state health authorities: official statements, linked to the original.

News reports never change a count. They can appear on an event page, clearly labeled "News report," and they can prompt us to look for an official statement.

What the words mean

We use WHO's international case definitions (revised in 2021):

  • Suspected: the illness and circumstances fit plague. This total includes probable and confirmed cases.
  • Probable: a suspected case with some lab evidence pointing to plague.
  • Confirmed: a lab test identified the plague bacterium, Yersinia pestis.
  • Deaths: as reported by the source. WHO counts deaths among all suspected cases, not only confirmed ones.
  • Cause undetermined: used for events like Irkutsk, where a death is real but no diagnosis has been confirmed. These are never added to plague totals.

Rules we follow

  1. Every number shows its source and date.
  2. We never add numbers from different sources together. WHO's worldwide totals and CDC's U.S. counts are shown side by side, not summed.
  3. "No data" is never shown as zero. Many countries, including the U.S. and Russia, sent WHO no plague data for 2019–2025.
  4. Provisional numbers (like CDC's weekly counts) are labeled provisional. When a final number replaces them, we use the final one and note the change.
  5. We keep saved copies of every source document, so our figures can be checked even if a web page changes.
  6. Before anything is published, a script checks every WHO number against the original report and against WHO's own printed totals. If one number doesn't match, nothing is published.

How often we update

We check WHO and CDC every day. CDC's U.S. counts change weekly. WHO publishes worldwide totals once a year, so there is no official live worldwide count for the current year; newer events appear under Outbreaks. Every change is listed on the Updates page.

Corrections

If we get something wrong, we fix it and record the correction on the Updates page with the date and what changed. To report an error, see About.