Communicable Diseases

What to remember

• A communicable disease spreads from a source to a susceptible host through a mode of transmission; breaking any link in this chain (source, route, host) controls the disease.
• Know each disease by its agent, vector or route, incubation period, key sign and main preventive measure; the vaccine-preventable diseases are controlled by the Universal Immunisation Programme with a strict cold chain.
• Mosquito-borne diseases differ by mosquito: Anopheles (malaria), Aedes (dengue, chikungunya), Culex (filariasis, Japanese encephalitis).

1. Basic terms

• Infection: entry and multiplication of an agent in the host. Infectious disease: a clinically visible disease caused by an infection.
• Incubation period: time from entry of the agent to the first sign or symptom.
• Reservoir: where the agent lives and multiplies (human, animal, soil). Source: the person or object that passes the infection directly.
• Carrier: an infected person without symptoms who can spread the agent (for example, typhoid carrier).
• Endemic: constantly present in an area. Epidemic: cases clearly more than expected in an area and time. Pandemic: epidemic across many countries. Sporadic: scattered cases. Zoonosis: disease passed from animals to humans (rabies).
• Isolation: separating sick persons. Quarantine: restricting healthy contacts for the longest incubation period. Surveillance: continuous collection and use of health data. Herd immunity: indirect protection of the unvaccinated when most of the community is immune.
• Disinfection: killing agents outside the body. Concurrent disinfection is done during illness; terminal disinfection is done after recovery or death.

Chain of infection: agent, reservoir, portal of exit, mode of transmission, portal of entry, susceptible host.

Modes of transmission: direct (touch, droplet, bite), indirect (water, food, air, vehicles, vectors, fomites) and vertical (mother to child).

Epidemiology formulas:

• Incidence rate = new cases in a period ÷ population at risk × 1,000.
• Attack rate = cases ÷ exposed persons × 100. Example: 12 ill among 60 persons who ate the food = 12 ÷ 60 × 100 = 20%.
• Case fatality rate = deaths from the disease ÷ cases of the disease × 100. Example: 3 deaths among 150 cases = 2%.
• Secondary attack rate shows spread among contacts of a case.

2. Disease summary table

3. Selected diseases in more detail

Tuberculosis. A person with cough for more than two weeks is a suspect. Sputum microscopy and molecular tests confirm. DOTS means a health worker watches the patient take the tablets. Drug-sensitive TB is treated with a multi-drug regimen. The Mantoux test shows past exposure, not active disease. BCG is given at birth.

Leprosy. Cardinal signs: a pale or red patch with loss of sensation, thickened nerve, and bacilli in a skin smear. Paucibacillary (PB) cases take multi-drug therapy for 6 months, and multibacillary (MB) cases for 12 months. The drugs are rifampicin, dapsone and clofazimine.

Malaria. Plasmodium falciparum causes the most severe form. The female Anopheles bites mostly at night and breeds in clean stagnant water. Rapid diagnostic test or blood smear confirms. Control: indoor residual spray, insecticide-treated nets, larvae-eating fish, and prompt treatment.

Dengue. The Aedes mosquito breeds in clean water stored in containers, coolers and tyres. Danger signs include bleeding, severe stomach pain, and vomiting. No specific drug; give fluids.

Diarrhoea and cholera. Oral rehydration solution (ORS) contains glucose, sodium chloride, potassium chloride and sodium citrate dissolved in 1 litre of clean water. Zinc is given for 14 days to children in diarrhoea. Worked example: one standard packet is mixed in 1 litre, so 5 litres of ORS need 5 packets. Do not add extra water or sugar.

Rabies. Wash the wound with soap and running water for 15 minutes, then give vaccine; immunoglobulin for severe (category III) exposure. Observe the biting dog for 10 days. Suturing the wound is avoided.

HIV. ELISA is the screening test; a confirmatory test follows. ART controls the virus. The ANM promotes HIV testing for every pregnant woman to prevent mother-to-child transmission. HIV does not spread through casual contact, shared meals, or mosquitoes.

Tetanus in newborns. Prevented by two doses of Td in pregnancy, clean delivery, and clean cord care.

4. Immunisation and the cold chain

Cold chain: ice-lined refrigerator keeps vaccines at +2 to +8 °C; deep freezer stores freezing-type vaccines at about -15 to -25 °C. Vaccines such as DPT, pentavalent, Td, TT and hepatitis B lose strength when frozen, so never freeze them. Check the vaccine vial monitor and expiry date and follow the open vial policy. Worked example: if 200 doses are issued and 150 are used, the wastage is 50 ÷ 200 × 100 = 25%.

A child is "fully immunised" when all due primary vaccines are received by 1 year. Immunisation coverage = children fully immunised ÷ children eligible × 100.

5. Prevention levels and the health worker's role

Primary prevention: vaccines, sanitation, vector control, health education. Secondary: early diagnosis and treatment, contact tracing. Tertiary: rehabilitation and disability limitation. In an outbreak the health assistant reports to the medical officer, carries out house-to-house search, chlorinates sources, and gives health education. Notification is required for diseases such as cholera and plague.

Exam traps

• Anopheles is for malaria, Aedes for dengue and chikungunya, and Culex for filariasis and Japanese encephalitis.
• Isolation is for the sick; quarantine is for healthy contacts.
• Hepatitis A spreads by the faecal-oral route; hepatitis B spreads by blood and body fluids.
• Mantoux shows exposure, not active TB; sputum test shows active TB.
• Incubation of dengue is short (days) but of leprosy is years.
• Epidemic means more than expected; endemic means usually present.
• PB leprosy takes 6 months of MDT; MB takes 12 months.
• Td is given to pregnant women and adolescents; DPT is given to infants.

One-liners

• Koplik's spots are seen in measles.
• A grey membrane in the throat suggests diphtheria.
• Widal test helps diagnose typhoid.
• Rice-water stools suggest cholera.
• Female Anopheles transmits malaria.
• Aedes aegypti bites in daytime.
• Lockjaw is a sign of tetanus.
• DOTS is the strategy for TB treatment.
• Rifampicin, dapsone and clofazimine are the leprosy MDT drugs.
• Wash a dog-bite wound for 15 minutes.
• Vaccine storage temperature is +2 to +8 °C.
• ELISA is the HIV screening test.

Practice questions

1. The time between entry of an infectious agent and the first symptom is called the

   1. serial interval
   2. latent period of immunity
   3. generation time
   4. incubation period
   Answer

   D. incubation period

   This interval is the incubation period.

2. A person who harbours an agent without symptoms and can spread it is a

   1. carrier
   2. vector
   3. fomite
   4. reservoir host only
   Answer

   A. carrier

   Such a person is a carrier, as in typhoid.

3. Restriction of movement of healthy contacts for the longest incubation period is

   1. disinfection
   2. surveillance
   3. isolation
   4. quarantine
   Answer

   D. quarantine

   Isolation is for the sick; quarantine is for healthy contacts.

4. A disease constantly present in a community is called

   1. pandemic
   2. endemic
   3. epidemic
   4. sporadic
   Answer

   B. endemic

   Endemic means usual presence in an area.

5. Malaria is transmitted by the bite of the female

   1. sandfly
   2. Anopheles mosquito
   3. Aedes mosquito
   4. Culex mosquito
   Answer

   B. Anopheles mosquito

   Plasmodium is carried by female Anopheles.

6. Dengue and chikungunya are spread by

   1. Culex quinquefasciatus
   2. Glossina fly
   3. Anopheles stephensi
   4. Aedes aegypti
   Answer

   D. Aedes aegypti

   Aedes aegypti is a day-biting mosquito that breeds in clean stored water.

7. Filariasis is transmitted by

   1. housefly
   2. Aedes mosquito
   3. sandfly
   4. Culex mosquito
   Answer

   D. Culex mosquito

   Wuchereria bancrofti is carried mainly by Culex.

8. Kala-azar is spread by the

   1. rat flea
   2. sandfly
   3. tick
   4. tsetse fly
   Answer

   B. sandfly

   Leishmania donovani is transmitted by sandflies.

9. Koplik's spots are seen in

   1. mumps
   2. diphtheria
   3. measles
   4. chickenpox
   Answer

   C. measles

   Koplik's spots appear inside the cheek in measles.

10. A grey membrane over the tonsils and throat is typical of

    1. whooping cough
    2. tetanus
    3. typhoid
    4. diphtheria
    Answer

    D. diphtheria

    This is the diphtheritic membrane.

11. Rice-water stools are characteristic of

    1. hepatitis A
    2. cholera
    3. typhoid
    4. dysentery
    Answer

    B. cholera

    Vibrio cholerae causes profuse watery stools.

12. The Widal test is used to support the diagnosis of

    1. typhoid
    2. tuberculosis
    3. leprosy
    4. dengue
    Answer

    A. typhoid

    Widal detects antibodies against Salmonella Typhi.

13. Lockjaw is a feature of

    1. rabies
    2. polio
    3. tetanus
    4. measles
    Answer

    C. tetanus

    Spasm of jaw muscles occurs in tetanus.

14. The causative agent of tuberculosis is

    1. Mycobacterium tuberculosis
    2. Clostridium tetani
    3. Mycobacterium leprae
    4. Corynebacterium diphtheriae
    Answer

    A. Mycobacterium tuberculosis

    Tuberculosis is caused by M. tuberculosis.

15. The multi-drug therapy for leprosy includes rifampicin, dapsone and

    1. pyrazinamide
    2. clofazimine
    3. streptomycin
    4. isoniazid
    Answer

    B. clofazimine

    MDT drugs are rifampicin, dapsone and clofazimine.

16. The Mantoux test indicates

    1. typhoid carrier state
    2. active leprosy
    3. HIV infection
    4. previous exposure to tubercle bacilli
    Answer

    D. previous exposure to tubercle bacilli

    A positive test shows exposure or infection, not necessarily active disease.

17. First step in managing a dog-bite wound is to

    1. suture it immediately
    2. apply turmeric
    3. wash it with soap and running water for 15 minutes
    4. cover it tightly without cleaning
    Answer

    C. wash it with soap and running water for 15 minutes

    Prolonged washing reduces virus at the site.

18. Hepatitis A mainly spreads by the

    1. faecal-oral route
    2. blood transfusion
    3. sexual contact only
    4. mother to child at birth
    Answer

    A. faecal-oral route

    Hepatitis A is food and water borne.

19. The screening test commonly used for HIV is

    1. Mantoux
    2. Schick
    3. Widal
    4. ELISA
    Answer

    D. ELISA

    ELISA screens; confirmation follows.

20. The recommended temperature range for storing vaccines in an ice-lined refrigerator is

    1. +10 °C to +15 °C
    2. -15 °C to -25 °C
    3. +2 °C to +8 °C
    4. -2 °C to +2 °C
    Answer

    C. +2 °C to +8 °C

    Vaccines are kept at +2 to +8 °C.

21. The intermediate amplifier host of Japanese encephalitis virus is the

    1. dog
    2. pig
    3. rat
    4. cow
    Answer

    B. pig

    Pigs amplify the virus; Culex spreads it.

22. Which vaccine is given at birth in the immunisation schedule?

    1. Pentavalent
    2. Td
    3. Measles-rubella
    4. BCG
    Answer

    D. BCG

    BCG is given at birth against TB.

23. Measles-rubella vaccine is first given at

    1. 16 years
    2. 10 years
    3. 9-12 months
    4. 6 weeks
    Answer

    C. 9-12 months

    MR is given at 9-12 months and again at 16-24 months.

24. Td vaccine is given to adolescents at the ages of

    1. 1 and 2 years
    2. 10 and 16 years
    3. 5 and 8 years
    4. 6 and 12 years
    Answer

    B. 10 and 16 years

    Td replaces TT at 10 and 16 years.

25. Which of the following vaccines is damaged by freezing?

    1. Pentavalent
    2. BCG
    3. Oral polio vaccine
    4. Measles vaccine
    Answer

    A. Pentavalent

    Adsorbed vaccines such as DPT, pentavalent, Td and hepatitis B must never be frozen.

26. Oral rehydration solution is prepared by dissolving one standard packet in

    1. 5 litres of water
    2. 1 litre of clean water
    3. 200 ml of water
    4. 500 ml of milk
    Answer

    B. 1 litre of clean water

    One standard ORS packet is mixed in 1 litre.

27. 12 of 60 persons who ate a meal fell ill. The attack rate is

    1. 20%
    2. 5%
    3. 12%
    4. 25%
    Answer

    A. 20%

    12 ÷ 60 × 100 = 20%.

28. There are 150 cases of a disease and 3 deaths. The case fatality rate is

    1. 0.2%
    2. 3%
    3. 2%
    4. 5%
    Answer

    C. 2%

    3 ÷ 150 × 100 = 2%.

29. A health centre issued 200 vaccine doses and used 150. The wastage rate is

    1. 33%
    2. 15%
    3. 50%
    4. 25%
    Answer

    D. 25%

    Wasted 50 ÷ 200 × 100 = 25%.

30. In a town of 40,000 there were 80 new cases of a disease in a year. The incidence per 1,000 is

    1. 0.2
    2. 8
    3. 20
    4. 2
    Answer

    D. 2

    80 ÷ 40,000 × 1,000 = 2.

31. A village has 500 children eligible for immunisation and 450 are fully immunised. Coverage is

    1. 90%
    2. 85%
    3. 45%
    4. 95%
    Answer

    A. 90%

    450 ÷ 500 × 100 = 90%.

32. A food outbreak affects 30 of 150 exposed people. The attack rate is

    1. 15%
    2. 20%
    3. 30%
    4. 5%
    Answer

    B. 20%

    30 ÷ 150 × 100 = 20%.

33. Of 80 patients with a disease, 4 die. The case fatality rate is

    1. 4%
    2. 20%
    3. 5%
    4. 8%
    Answer

    C. 5%

    4 ÷ 80 × 100 = 5%.

34. A leprosy patient with a multibacillary form needs multi-drug therapy for how many months?

    1. 12
    2. 3
    3. 6
    4. 24
    Answer

    A. 12

    MB cases take 12 months; PB cases take 6 months.

35. Statements: 1. Isolation is applied to sick persons. 2. Quarantine is applied to healthy contacts. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both definitions are correct.

36. Statements: 1. Hepatitis B spreads through blood and body fluids. 2. Hepatitis A spreads mainly through contaminated food and water. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    These are the main routes.

37. Statements: 1. The Mantoux test confirms active tuberculosis. 2. BCG vaccine is given at birth. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Mantoux shows exposure only; BCG is given at birth.

38. Statements: 1. Aedes aegypti bites mainly at night. 2. Female Anopheles transmits malaria. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Aedes aegypti bites in the daytime.

39. Statements: 1. DPT vaccine may be frozen safely. 2. Vaccines are stored at +2 to +8 °C in the ice-lined refrigerator. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    DPT is freeze-sensitive and must not be frozen.

40. Statements: 1. HIV spreads by sharing meals. 2. HIV can pass from mother to child. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    HIV does not spread by casual contact such as sharing meals.

41. Match: (a) cholera, (b) malaria, (c) filariasis, (d) rabies with (i) Culex, (ii) Vibrio, (iii) dog bite, (iv) Plasmodium. Choose the correct pairing.

    1. a-iv, b-ii, c-i, d-iii
    2. a-ii, b-i, c-iv, d-iii
    3. a-iii, b-iv, c-i, d-ii
    4. a-ii, b-iv, c-i, d-iii
    Answer

    D. a-ii, b-iv, c-i, d-iii

    Cholera-Vibrio, malaria-Plasmodium, filariasis-Culex, rabies-dog bite.

42. Match the disease with the typical sign: (a) measles, (b) diphtheria, (c) tetanus, (d) typhoid.

    1. a-Koplik's spots, b-membrane, c-lockjaw, d-step-ladder fever
    2. a-step-ladder fever, b-membrane, c-lockjaw, d-Koplik's spots
    3. a-membrane, b-Koplik's spots, c-lockjaw, d-step-ladder fever
    4. a-Koplik's spots, b-lockjaw, c-membrane, d-step-ladder fever
    Answer

    A. a-Koplik's spots, b-membrane, c-lockjaw, d-step-ladder fever

    Each sign belongs to the disease listed in the first option.

43. Which disease is correctly matched with its agent?

    1. Diphtheria - Clostridium tetani
    2. Typhoid - Vibrio cholerae
    3. Whooping cough - Salmonella Typhi
    4. Leprosy - Mycobacterium leprae
    Answer

    D. Leprosy - Mycobacterium leprae

    Only the leprosy pair is correct.

44. A child with cough for more than two weeks, weight loss and evening fever should first be tested for

    1. measles
    2. tetanus
    3. mumps
    4. tuberculosis
    Answer

    D. tuberculosis

    These features suggest tuberculosis; sputum testing is advised.

45. Which measure best prevents neonatal tetanus?

    1. Giving vitamin A at birth
    2. Giving antibiotics to all newborns
    3. Two Td doses in pregnancy and clean cord care
    4. Avoiding breastfeeding
    Answer

    C. Two Td doses in pregnancy and clean cord care

    Maternal immunisation and a clean delivery protect the newborn.