CDS (II) 2021 General Knowledge · Question 97 of 120

Correct — A, Endemic. In epidemiology an infection is called endemic when it is constantly present, or maintained at a baseline level, in a given population or geographic area — the case numbers are steady and predictable, and they stay that way without being topped up from outside. The condition behind that steadiness is easy to state: for a disease spread person to person, the basic reproduction number multiplied by the proportion of the population still susceptible has to sit at one, so that each infection replaces itself exactly and the infection neither dies out nor grows exponentially. The wording of the stem — a baseline level, in a geographic area, without external input — is that definition almost word for word. The other three options all describe departures from the steady state rather than the steady state itself.

• (b)Pandemic — A pandemic is an epidemic that has spread over a very large part of the world, generally across many countries or continents. It is defined by geographical reach, and it is by definition not a steady baseline.
• (c)Epidemic — An epidemic is a new disease spreading, or an already endemic disease whose infection rate has risen significantly above what is expected. It is the opposite of the situation described — case numbers are climbing, not holding.
• (d)Outbreak — An outbreak is an epidemic confined to a small area, such as a single town, an institution or a single event. Again a rise above the expected level, and again not a baseline maintained over time.

The four words form a ladder of scale and of departure from normal. Sporadic cases appear occasionally with no pattern. Endemic means constant presence at a predictable level. An outbreak is a localised rise above that level, an epidemic a larger and sustained one, and a pandemic an epidemic that has crossed continents. The words say nothing at all about severity, which is the point most often misread.

This paper was written in the second year of Covid-19, when the question of whether the disease would settle into an endemic pattern was being argued about in public every week, so the vocabulary was live. The trap the question sets is the everyday sense of 'endemic' as small or harmless. It carries no such meaning. An endemic disease has a steady, predictable number of cases, but that number can be very high and the illness can be severe — malaria and tuberculosis are endemic in parts of India and are among the country's heaviest disease burdens. What changed after this exam is the label attached to Covid-19 rather than the definitions themselves, which are stable.

• An infection is endemic when it is constantly present, or maintained at a baseline level, in a population or area.
• For person-to-person spread the endemic steady state requires the basic reproduction number multiplied by the susceptible fraction to equal one, so cases neither die out nor grow exponentially.
• An outbreak is an epidemic restricted to a small area; an epidemic is a significant rise above the expected rate; a pandemic is an epidemic spread over a very large part of the world.
• Endemic says nothing about severity or numbers — an endemic disease can have high case counts and be severe.
• A disease that is endemic in one region may be absent from another, so the label always attaches to a place as well as to a pathogen.

• Reading 'endemic' as mild or rare. It means steady, not small.
• Confusing endemic in epidemiology with endemic in ecology, where it means a species found only in one region.
• Assuming pandemic means severe; the word describes geographical spread, not lethality.

As a definition item on one of these four words, or as a match between the four terms and short descriptions.

• practice — not a real PYQ

  An epidemic that has spread across several countries and continents is described as a

  • (a)cluster
  • (b)pandemic
  • (c)sporadic occurrence
  • (d)endemic infection

  Answer(b) pandemic — the word refers to geographical spread, not to how severe the disease is.

• practice — not a real PYQ

  Which one of the following statements about an endemic disease is correct?

  • (a)It is always mild and causes few deaths
  • (b)It occurs only once in a population and then disappears
  • (c)It is constantly present at a steady, predictable level in a given area
  • (d)It affects several continents at the same time

  Answer(c) It is constantly present at a steady, predictable level in a given area — the level itself may be high or low, and the disease may be mild or severe.