Which one of the following diseases is not a bacterial disease ?

Correct — option (4), Dengue.

The stem asks which disease is not bacterial. The World Health Organization's fact sheet describes dengue as "a viral infection caused by the dengue virus (DENV)", transmitted to humans through the bite of infected mosquitoes.

The mosquito is the carrier, not the cause. WHO names infected female Aedes aegypti as the primary vector, with other Aedes species playing a secondary part. The disease itself comes from the virus.

The other three options all have bacterial causes: diphtheria and plague are caused by bacteria, and bacteria are among the causes of pneumonia. So Dengue is the option with no bacterial cause.

The idea to remember: dengue is a viral disease spread by Aedes mosquitoes; diphtheria and plague are bacterial; pneumonia can be bacterial, viral or fungal.

Why the others are wrong

• (1)
  Diphtheria — Diphtheria is a bacterial disease. The US Centers for Disease Control and Prevention (CDC) attributes it to a toxin produced by some strains of the bacterium Corynebacterium diphtheriae.

  WHO says the disease affects the upper respiratory tract and, less often, the skin, and that the toxin damages the heart and the nerves. It is vaccine-preventable, with multiple and booster doses needed.

• (2)
  Plague — Plague is a bacterial disease caused by Yersinia pestis. WHO describes Y. pestis as a zoonotic bacterium usually found in small mammals and their fleas.

  Bubonic plague, the most common form, follows the bite of an infected flea; the pneumonic form spreads from person to person through droplets. WHO says antibiotic treatment is effective against plague bacteria, so early diagnosis and treatment can save lives.

• (3)
  Pneumonia — Pneumonia names an infection of the lungs, and WHO lists viruses, bacteria and fungi among its causes. It is not purely bacterial, but bacteria are among its most common causes.

  WHO names Streptococcus pneumoniae as the most common cause of bacterial pneumonia in children and Haemophilus influenzae type b (Hib) as the second. Dengue, by contrast, has only a viral cause.

Infectious diseases are grouped by the kind of organism that causes them: bacteria, viruses, fungi or parasites. The name of a disease does not always point to one kind.

Some diseases have a single causative organism. Dengue is caused by the dengue virus, plague by the bacterium Yersinia pestis, typhoid by Salmonella Typhi and chikungunya by the chikungunya virus.

Others are named by the organ they affect. Pneumonia is an infection of the lungs that viruses, bacteria or fungi can cause. A vector, such as a mosquito or flea, carries a pathogen but is not the pathogen.

Key facts

• WHO: dengue is a viral infection caused by the dengue virus (DENV), spread mainly by infected female Aedes aegypti mosquitoes.
• WHO: plague is caused by Yersinia pestis, a zoonotic bacterium usually found in small mammals and their fleas.
• CDC: diphtheria is caused by a toxin produced by some strains of the bacterium Corynebacterium diphtheriae.
• WHO: pneumonia can be caused by viruses, bacteria or fungi; Streptococcus pneumoniae is the most common cause of bacterial pneumonia in children.
• WHO: respiratory syncytial virus is the most common viral cause of pneumonia; Hib is the second most common bacterial cause.

Common traps

• Pneumonia is named for the organ, not the organism. Viruses, bacteria and fungi can all cause it, so do not mark it "viral" or "bacterial" without reading what the option set allows.
• The vector is not the pathogen. Aedes mosquitoes carry dengue, but the cause is a virus; fleas carry plague, but the cause is a bacterium.
• Diphtheria's harm comes from a toxin, yet the disease is bacterial: the toxin is made by strains of Corynebacterium diphtheriae.

• practice — not a real PYQ

  Which one of the following diseases is caused by a virus?

  • (a)Typhoid
  • (b)Tuberculosis
  • (c)Chikungunya
  • (d)Plague
  Answer(3) — WHO describes chikungunya as a disease caused by the chikungunya virus. Typhoid is caused by the bacterium Salmonella Typhi, tuberculosis by bacteria, and plague by the bacterium Yersinia pestis, so options (1), (2) and (4) are bacterial.
• practice — not a real PYQ

  Which one of the following pairs of disease and type of pathogen is NOT correctly matched?

  • (a)Plague – Bacterium
  • (b)Diphtheria – Bacterium
  • (c)Dengue – Bacterium
  • (d)Chikungunya – Virus
  Answer(3) — Dengue is caused by the dengue virus, not a bacterium. Plague (Yersinia pestis) and diphtheria (Corynebacterium diphtheriae) are bacterial, and chikungunya is caused by the chikungunya virus, so options (1), (2) and (4) are correctly matched.