Ecology, epidemiology and control of infectious diseases 2025-2026
ECOLOGY, EPIDEMIOLOGY AND CONTROL OF INFECTIOUS DISEASES
CHAPTER 1: POPULATION ECOLOGY
WHAT IS A POPULATION?
= A group of individuals of one species, living at a certain place in space and time.
• If they don’t belong to the same species, we call it a community.
• A certain place is not easy to define, and time is a moving window. This is a di@icult definition, so we often
use another definition.
= A group of individuals of one species under investigation.
DETERMINING POPULATION SIZE
LIMITS OF THE POPULATION
• To determine the population size, we need to know the limits of a population.
o Geographical?
o Social?
o Connectivity?
o Time?
• Identifying the individuals that belong to the population can be di@icult.
o It’s possible that you live at the border and commute to France or live in Bruxelles and commute to
Paris. This has an e@ect on infectious diseases as well. Do we need to broaden the definition of a
population?
o Birds can move from one forest to another. This is also true for parasites.
• Parasite populations:
o Hosts form a population and the parasites form populations
o Parasites can live in di@erent hosts
o Infrapopulation: population of parasites in 1 host
§ E.g. an elephant is infected by red parasites
o Component population: population of parasites in 1 life stage
§ E.g. snakes are infected by the same parasites, but by a
di@erent stage of those parasites. We can ask ourselves
whether this is also the same population then?
o Suprapopulation: population of parasites over all the hosts
§ E.g. Many elephants are all infected by the same parasites,
but there are also gira@es that are infected with those parasites.
ABUNDANCE
• = The number of organisms in a population, combining ‘intensity’ (density within inhabited areas) and
‘prevalence’ (number and size of inhabited areas).
o Intensity = the number of organisms that live in a particular patch
o Prevalence for parasitic infections = proportion of hosts that are infected with a certain parasite
• Absolute number!
• This doesn’t tell you anything about the contact with other individuals or food per individual.
• E@ects of abundance on transmission of infections among humans and from reservoirs/vectors to
humans:
o Direct:
§ Higher abundance → more sources of infection.
§ Higher abundance → higher prevalence (often).
o Indirect:
§ E@ects on other hosts/vectors.
§ E@ects on infrastructure, health system, food, …
DENSITY
• = The number of individuals per area (or more general: per unit of resource).
o e.g. nr of mice that live in one hectare.
• Density is an abstraction (but maybe more relevant for the individual that experiences it)
o E.g. Imagine our class in a college room. Then it is not really dens but in a normal classroom it would
feel very dense.
1
,Ecology, epidemiology and control of infectious diseases 2025-2026
COUNTING WHICH INDIVIDUALS?
This depends on your research question or hypothesis.
Example: How many frogs are in this picture?
⇨ Possible that it is 2 but there are also frog eggs, which are important for growth in the
population. Eggs can also be seen as individuals that will develop in the future.
COUNTING PARASITES
Prevalence: proportion of all the individuals that are
infected versus the total. For the yellow faces that is 4
infected versus 10 individuals, so 40%. For the red ones, 6
infected versus 10 individuals in total.
Intensity: count the number of parasites that infect each
infected individual.
Mean intensity: average of the intensity
Abundance: count the number of parasites that infect
each (infected or non infected) individual.
Mean abundance: Average of the abundance.
POPULATION DYNAMICS
GENERAL
We take into account the influence of birth, death, immigration and emigration on our population size.
𝑁!"# = 𝑁$%&! + 𝐵 − 𝐷 + 𝐼 − 𝐸
NATALITY OR BIRTH
Mainly dependent on females (in most sexually reproducing species)
• Number: How many females are there?
• Age: How old are the females?
• Reproductive capacity
⇨ Realised reproductive capacity
We don’t really look at the maximum reproductive capacity but more the realised reproductive capacity. So, not like a
woman can have X kids, but the mean for Belgium is 1.5 kids per woman.
Our population would actually decrease, but the population in Belgium is still increasing because of immigration.
MORTALITY
Dependent on many factors.
• E@ects depending on which part of the population dies.
o Survivorship curves
o Population composition
• The maximum lifespan is only rarely relevant. The realized lifespan is more relevant and often shorter than
the maximum lifespan. It’s only relevant if we want to make demographic models.
2
,Ecology, epidemiology and control of infectious diseases 2025-2026
Types of survival (survivorship curves)
Type 1: Populations where most individuals can live up to a certain age and then
after they will almost all die at once (e.g. elephants)
Type 2: Organisms die at a constant rate.
Type 3: Populations where individuals will most likely die early, but the ones that
survive can survive for a very long time (e.g. frogs: many eggs die and only several
can survive)
Di4erent populations have di4erent characteristics
Almost 70% of the children in the 17th century England died before the age of 5 (lack of food, infections…)
Population composition
Depending on the situation, populations will have a di@erent survivorship, so it’s important to understand the
agestructure of a population.
• Young vs old: The older population will not contribute as much to the next population (reproduction) as the
younger population. So depending on when a person dies, this has more or less e@ect on the population.
• Female vs male: important for reproductive capacity
Age distributions di@er at di@erent levels: within and between populations. E.g. human age pyramids.
Belgium: Now there are less children born than in the 90s, so now the younger population must contribute more to the pensions etc.
IMMIGRATION AND EMIGRATION
• Very di@icult to study in animal populations
• Usually assumed that immigration = emigration ⇨ faulty assumption, will be explained later
• These are very important for parasite ecology!
o Transmission between hosts
§ Emigration: leaving of one host - Immigration: in to another host
§ Every infection is an act of immigration for one host and emigration from another host
o Invasion of infection in new area
§ When immigrants arrive, does the infection spread?
POPULATION GROWTH
The di@erence in your population over a certain time di@erence = births – deaths
∆𝑁
=𝐵−𝐷
∆𝑡
This change in your population is also called R.
Net reproductive rate R = population growth rate 𝜆
'!"#
𝑅= '!
and 𝑁$() = 𝑅𝑁$
Basic reproductive rate R0 = Rnought
𝑁$ = 𝑅$ 𝑁* = 𝑅* 𝑁* with t = generation time
So R0 is the factor, on average, with which an individual replaces itself over the
course of a generation.
If the population is doubled, then R0 = 2.
R0 > 1: Population will grow
R0 < 1: Population will decrease
3
, Ecology, epidemiology and control of infectious diseases 2025-2026
Expressed with birth and death rate N = initial population size
+'
Here, it is instantaneously measured with di@erentials. +$ = (𝑏 − 𝑑)𝑁 b = births
d = deaths
Intrinsic rate of natural increase r
This is exactly at a certain point in time what your population growth will be. Critical value of r = 0;
𝑑𝑁 positive = growth,
= 𝑟𝑁 negative = decrease
𝑑𝑡
Exponential growth
• population continues growing
• Does not happen a lot in real life
• E.g. bacteria have exponential growth until human dies or immune system
suppresses the infection
Logistic growth (sigmoid growth)
• Resources are limited (“carrying capacity”)
• The more animals, the more di@icult for individual animals: density dependent mortality
• The population growth will reach an equilibrium (as you can see in the curve)
+' ,-'
+$
= 𝑟𝑁 4 , 5
o K = carrying capacity
o If the population (N) is very small, then the fraction will be around 1
o By the end, N will be as big as K ⇨ rN = 0 ⇨ equal amount of births and
deaths, no growth of population
Density dependent population regulation
Where birth and death rate cross = population equilibrium
Density dependence is also relevant for parasites!
• The more parasites the sheep has, the lower the mean egg
output/parasite/day. So the reproductive outcome of each
individual parasite is less, but the total amount of eggs is increased!
• If there are more worms (size of infection increases), the worms get
smaller, but the total burden (and weight) of the parasite stays the
same.
DIFFERENT TYPES OF POPULATION DYNAMICS
• Regular (deterministic interactions)
• Irregular (stochastic factors, random chance)
Stable cycles: small
rodent populations
When it gets close to
carrying capacity, then
it can crash because
there are not enough
resources or there are
weather events.
4
ECOLOGY, EPIDEMIOLOGY AND CONTROL OF INFECTIOUS DISEASES
CHAPTER 1: POPULATION ECOLOGY
WHAT IS A POPULATION?
= A group of individuals of one species, living at a certain place in space and time.
• If they don’t belong to the same species, we call it a community.
• A certain place is not easy to define, and time is a moving window. This is a di@icult definition, so we often
use another definition.
= A group of individuals of one species under investigation.
DETERMINING POPULATION SIZE
LIMITS OF THE POPULATION
• To determine the population size, we need to know the limits of a population.
o Geographical?
o Social?
o Connectivity?
o Time?
• Identifying the individuals that belong to the population can be di@icult.
o It’s possible that you live at the border and commute to France or live in Bruxelles and commute to
Paris. This has an e@ect on infectious diseases as well. Do we need to broaden the definition of a
population?
o Birds can move from one forest to another. This is also true for parasites.
• Parasite populations:
o Hosts form a population and the parasites form populations
o Parasites can live in di@erent hosts
o Infrapopulation: population of parasites in 1 host
§ E.g. an elephant is infected by red parasites
o Component population: population of parasites in 1 life stage
§ E.g. snakes are infected by the same parasites, but by a
di@erent stage of those parasites. We can ask ourselves
whether this is also the same population then?
o Suprapopulation: population of parasites over all the hosts
§ E.g. Many elephants are all infected by the same parasites,
but there are also gira@es that are infected with those parasites.
ABUNDANCE
• = The number of organisms in a population, combining ‘intensity’ (density within inhabited areas) and
‘prevalence’ (number and size of inhabited areas).
o Intensity = the number of organisms that live in a particular patch
o Prevalence for parasitic infections = proportion of hosts that are infected with a certain parasite
• Absolute number!
• This doesn’t tell you anything about the contact with other individuals or food per individual.
• E@ects of abundance on transmission of infections among humans and from reservoirs/vectors to
humans:
o Direct:
§ Higher abundance → more sources of infection.
§ Higher abundance → higher prevalence (often).
o Indirect:
§ E@ects on other hosts/vectors.
§ E@ects on infrastructure, health system, food, …
DENSITY
• = The number of individuals per area (or more general: per unit of resource).
o e.g. nr of mice that live in one hectare.
• Density is an abstraction (but maybe more relevant for the individual that experiences it)
o E.g. Imagine our class in a college room. Then it is not really dens but in a normal classroom it would
feel very dense.
1
,Ecology, epidemiology and control of infectious diseases 2025-2026
COUNTING WHICH INDIVIDUALS?
This depends on your research question or hypothesis.
Example: How many frogs are in this picture?
⇨ Possible that it is 2 but there are also frog eggs, which are important for growth in the
population. Eggs can also be seen as individuals that will develop in the future.
COUNTING PARASITES
Prevalence: proportion of all the individuals that are
infected versus the total. For the yellow faces that is 4
infected versus 10 individuals, so 40%. For the red ones, 6
infected versus 10 individuals in total.
Intensity: count the number of parasites that infect each
infected individual.
Mean intensity: average of the intensity
Abundance: count the number of parasites that infect
each (infected or non infected) individual.
Mean abundance: Average of the abundance.
POPULATION DYNAMICS
GENERAL
We take into account the influence of birth, death, immigration and emigration on our population size.
𝑁!"# = 𝑁$%&! + 𝐵 − 𝐷 + 𝐼 − 𝐸
NATALITY OR BIRTH
Mainly dependent on females (in most sexually reproducing species)
• Number: How many females are there?
• Age: How old are the females?
• Reproductive capacity
⇨ Realised reproductive capacity
We don’t really look at the maximum reproductive capacity but more the realised reproductive capacity. So, not like a
woman can have X kids, but the mean for Belgium is 1.5 kids per woman.
Our population would actually decrease, but the population in Belgium is still increasing because of immigration.
MORTALITY
Dependent on many factors.
• E@ects depending on which part of the population dies.
o Survivorship curves
o Population composition
• The maximum lifespan is only rarely relevant. The realized lifespan is more relevant and often shorter than
the maximum lifespan. It’s only relevant if we want to make demographic models.
2
,Ecology, epidemiology and control of infectious diseases 2025-2026
Types of survival (survivorship curves)
Type 1: Populations where most individuals can live up to a certain age and then
after they will almost all die at once (e.g. elephants)
Type 2: Organisms die at a constant rate.
Type 3: Populations where individuals will most likely die early, but the ones that
survive can survive for a very long time (e.g. frogs: many eggs die and only several
can survive)
Di4erent populations have di4erent characteristics
Almost 70% of the children in the 17th century England died before the age of 5 (lack of food, infections…)
Population composition
Depending on the situation, populations will have a di@erent survivorship, so it’s important to understand the
agestructure of a population.
• Young vs old: The older population will not contribute as much to the next population (reproduction) as the
younger population. So depending on when a person dies, this has more or less e@ect on the population.
• Female vs male: important for reproductive capacity
Age distributions di@er at di@erent levels: within and between populations. E.g. human age pyramids.
Belgium: Now there are less children born than in the 90s, so now the younger population must contribute more to the pensions etc.
IMMIGRATION AND EMIGRATION
• Very di@icult to study in animal populations
• Usually assumed that immigration = emigration ⇨ faulty assumption, will be explained later
• These are very important for parasite ecology!
o Transmission between hosts
§ Emigration: leaving of one host - Immigration: in to another host
§ Every infection is an act of immigration for one host and emigration from another host
o Invasion of infection in new area
§ When immigrants arrive, does the infection spread?
POPULATION GROWTH
The di@erence in your population over a certain time di@erence = births – deaths
∆𝑁
=𝐵−𝐷
∆𝑡
This change in your population is also called R.
Net reproductive rate R = population growth rate 𝜆
'!"#
𝑅= '!
and 𝑁$() = 𝑅𝑁$
Basic reproductive rate R0 = Rnought
𝑁$ = 𝑅$ 𝑁* = 𝑅* 𝑁* with t = generation time
So R0 is the factor, on average, with which an individual replaces itself over the
course of a generation.
If the population is doubled, then R0 = 2.
R0 > 1: Population will grow
R0 < 1: Population will decrease
3
, Ecology, epidemiology and control of infectious diseases 2025-2026
Expressed with birth and death rate N = initial population size
+'
Here, it is instantaneously measured with di@erentials. +$ = (𝑏 − 𝑑)𝑁 b = births
d = deaths
Intrinsic rate of natural increase r
This is exactly at a certain point in time what your population growth will be. Critical value of r = 0;
𝑑𝑁 positive = growth,
= 𝑟𝑁 negative = decrease
𝑑𝑡
Exponential growth
• population continues growing
• Does not happen a lot in real life
• E.g. bacteria have exponential growth until human dies or immune system
suppresses the infection
Logistic growth (sigmoid growth)
• Resources are limited (“carrying capacity”)
• The more animals, the more di@icult for individual animals: density dependent mortality
• The population growth will reach an equilibrium (as you can see in the curve)
+' ,-'
+$
= 𝑟𝑁 4 , 5
o K = carrying capacity
o If the population (N) is very small, then the fraction will be around 1
o By the end, N will be as big as K ⇨ rN = 0 ⇨ equal amount of births and
deaths, no growth of population
Density dependent population regulation
Where birth and death rate cross = population equilibrium
Density dependence is also relevant for parasites!
• The more parasites the sheep has, the lower the mean egg
output/parasite/day. So the reproductive outcome of each
individual parasite is less, but the total amount of eggs is increased!
• If there are more worms (size of infection increases), the worms get
smaller, but the total burden (and weight) of the parasite stays the
same.
DIFFERENT TYPES OF POPULATION DYNAMICS
• Regular (deterministic interactions)
• Irregular (stochastic factors, random chance)
Stable cycles: small
rodent populations
When it gets close to
carrying capacity, then
it can crash because
there are not enough
resources or there are
weather events.
4
