Spread of Disease
Learning intention
•the spread of a specific disease involves a range of interrelated factors, including
o growth of the pathogen population
o density of the host population
o mode of transmission
Success criteria
- Explain how population growth, host population density and mode of transmission
influence the ability of a disease to spread
Review Questions
1. What is the three characteristic that all protists share?
2. What are the 3 phases in the lifecycle of malaria?
3. In which host are the plasmodia reproducing sexually? Asexually?
As human civilisation grows and becomes more urbanised and
international travel increases more deadly pandemics will occur!
Humans live in higher density, encroach into animal habitats and carry
diseases to other continents!
Describing the Occurrence of Infections
Endemic- prevalent at constant rate within a population
Sporadic- diseases that occur irregularly within a
population
Epidemic- an increase in cases of a disease over what is
considered normal
Pandemic- epidemic that spreads across multiple
continents
Pathogen Characteristics and Survival
• Infectivity
• the ability of a pathogen to be transmitted between hosts and multiply
• The more easily the pathogen can be passed on the more likely it is to survive
• Virulence- the ability of a pathogen to cause severe disease in the
host (may cause population decline)
• Ability to live outside host- spores, fomites
• Being asymptomatic while contagious (or displaying symptoms)
Factors Affecting the Spread of Disease
1. Growth of the pathogen population
2. Density of host population
3. Mode of Transmission
1. Growth of pathogen Population
• Increased size of pathogen population=increased risk of transmission
• Favourable environmental conditions can shorten reproductive cycle
increasing population size
• Eg in the right conditions some bacteria cn undergo binary fission
every 20 minutes
2. Density of the Host Population
• The more densely populated a community is the more likely
organisms are to come into contact with an infected organism
• Solitary organisms v grouping organisms
• Zoonoses are more likely as reduced habitat leads to greater
interactions between humans and wild animals
3. Mode of Transmission
• Indirect (Vector, airborne & fomites, soil/waterborne)
• Direct (Direct contact, close contact, reservoir)
• Multiple modes of transmission increases likely hood of transmission
(eg COVID-19)
2018 38b
• Discuss how population density and the provision of healthcare can
influence the susceptibility of an urban area to an influenza epidemic
(10 marks)
How population density can influence susceptibility of an urban area to an influenza epidemic
Either (5 marks)
• an epidemic is where many individuals in a region/area are infected
• high population density increases susceptibility (to epidemic)
• influenza is spread through close contact
• higher population density/more people in an area
• more often infected people will come into contact with
• uninfected/susceptible people
• (therefore) influenza will spread faster in high density areas
Or the converse
Biology WA
• P450-4
• Q1-3, 5 p458