S7) Infection Prevention

Question 1

How do viruses cause infections?

Answer 1

⇒ Encounter a host cell

⇒ Enter host cell

⇒ Undergo primary replication

⇒ Spread to a final target tissue

⇒ Infect and successfully replicate in a susceptible population of host cells

Question 2

What causes the signs and symptoms seen in viral infections?

Answer 2

The signs and symptoms of viral disease are the culmination of a series of interactions between the virus and the host

Question 3

Describe the underlying process in acute infections

Answer 3

Acute infections involve a virus undergoing multiple rounds of replication, resulting in the death of the host cell, which is used as a factory for virus production

E.g. poliovirus, influenza virus

Question 4

Describe the underlying process occurring in latent infections

Answer 4

• Latent infections involve persisting viral DNA either as an extrachromosomal element / as an integrated sequence within the host genome
• During cell growth, the genome of the virus is replicated along with the chromosomes of the host cell
• E.g. herpes simplex virus type 1 (DNA viruses/retroviruses)*

Question 5

Describe the underlying process occurring in chronic infections

Answer 5

• Chronic infection involves the continuous shedding of viral particles after the period of acute illness, sometimes without host cell death or overt cellular injury
• They don’t result in overt disease but produce disease after a prolonged interval and are associated with defective host immune responses
• E.g. hepatitis C virus → chronic hepatitis → liver cancer*

Question 6

Explain the three different ways in which infections can be transmitted

Answer 6

• Infections are transmissible from a non-human source to humans e.g. food/water – food poisoning organisms, animals – rabies
• Infections are transmissible from person to person directly e.g. norovirus, influenza, N.gonorrhoea
• Infections are transmissible from person to person indirectly (vector) e.g. mosquitoes – malaria

Question 7

In terms of infection transmission and prevention, define the following terms:

• Endemic disease
• Outbreak

Answer 7

• Endemic disease – the usual background rate
• Outbreak – two/more cases linked in time and place (plausible in terms of infection dynamics – incubation period, etc)

Question 8

In terms of infection transmission and prevention, define the following terms:

• Epidemic
• Pandemic

Answer 8

• Epidemic – a rate of infection greater than the usual background rate
• Pandemic – very high rate of infection spreading across many regions, countries, continents

Question 9

In terms of the influenza virus, distinguish between antigenic drift and antigenic shift

Answer 9

• Antigenic drift refers to the changes to the flu virus that happen slowly over time
• Antigenic shift refers to the event when two different flu strains combine to infect the same cell

Question 10

What is basic reproduction number (R₀) and what is its significance?

Answer 10

R₀ is the average number of cases one case generates over a course of its infectious period, in an otherwise uninfected, non-immune population:

• If R₀ > 1 → increase in cases
• If R₀ = 1 → stable number of cases
• If R₀ < 1 → decrease in cases

1 - 1/R0 = determines how much of the population need to be vaccinated for herd immunity to work

eg if R0 = 3

1 - 1/3 = 66% of pop need to be vaccinated

Question 11

Provide 3 reasons for outbreaks, epidemics and pandemics

Answer 11

• New pathogen (antigens, virulence factors, antibacterial resistance)
• New hosts (non-immunes, healthcare effects)
• New practice (social, healthcare)

Question 12

What is infectious dose?

Answer 12

• Infectious dose is the number of micro-organisms required to cause infection
• It varies by the micro-organism, presentation of micro-organism and immunity of potential host

Question 13

Outline infection prevention interventions in terms of the 4 P’s

Answer 13

Question 14

Outline infection prevention interventions in terms of pathogen

Answer 14

• Reduce/eradicate pathogen: antibacterials, decontamination, sterilisation
• Reduce/eradicate vector: eliminate vector breeding sites

Question 15

Outline infection prevention interventions in terms of patient

Answer 15

• Improved health: nutrition, medical treatment
• Immunity: passive e.g. maternal antibody, active e.g. vaccination

Question 16

Outline infection prevention interventions in terms of practice

Answer 16

• Avoidance of pathogen or its vector
• Protective clothing, equipment: long sleeves & trousers against mosquito bites, PPE in hospitals
• Behavioural: safe sex, food and drink preparation, safe disposal of sharps

Question 17

Outline infection prevention interventions in terms of place

Answer 17

Environmental engineering:

• Safe water
• Safe air
• Good quality housing
• Well-designed healthcare facilities

Question 18

Explain the good and bad consequences of infection prevention

Answer 18

• Good – decreased incidence or elimination of disease/organism e.g. smallpox, polio
• Bad:

I. Decreased exposure to pathogen → decreased immune stimulus → decreased antibody → increased susceptibles → outbreak

II. Later average age of exposure → increased severity

Question 19

What is the Epstein-Barr virus?

Answer 19

EBV infection (aka infectious mononucleosis) is a viral latent infection which affects B lymphocytes and produces characteristic atypical lymphocytes that may be useful in the diagnosis of the disease

Question 20

examples of person - person transmission

Answer 20

influenza

covid 19

norovirus

gonoeehoea

Question 21

indirect person - person transmission

Answer 21

malaria

Question 22

epidemic curve

Answer 22

Question 23

stochastic nature of small scale outbreaks

Answer 23

outbreaks are very random

Question 24

definition of surveillance

Answer 24

• what is happening now with infection and what might happen