AEC 360 Ecology
Lecture 21: The Ecology, Evolution, and
Behavior of Infectious Disease
I. Types of interactions between species
A. Interactions can be classified based on
whether they have positive, negative, or no effect on the other
species population size or growth rate
that have a positive effect on both species are called mutualisms
that have a positive effect on one species and a negative effect on
the other are called a variety of names, including predation,
disease, herbivory, parasitism
that have negative effects on both species are called competition
do we study infectious disease?
a. >25% of annual human
deaths worldwide are the direct result of infectious disease
is infectious disease?
a. What is a parasite or
b. What is infection?
c. What is a host?
d. Examples of
infectious disease and their pathogens.
e. Disease triangle
f. Community ecology of
g. What infectious
disease is not…
1. Non Infectious disease
i. Doesn't necessarily kill its victim
After infection, victim often becomes immune
Disease organism is much smaller than its victim
iv. Disease organism is often not independently mobile
IV. How do we study infectious disease?
A. Population ecology of
B. SIR Models
1. Rate of transmission
-- The process by which
a pathogen passes from a source of infection to a new host and
infects that host
-- Dependent on rate of
contact and probability of infection given contact.
2. Why is it important?
-- Host individuals are
-- Hosts defend
-- Hosts die (especially
Modes of transmission
4. R0 – the basic reproductive ratio (Pages 356-357)
a. # secondary infections produced
by a single infected host in a susceptible population
b. Depends on:
-- Rate of transmission (b)
-- Rate of removal (g)
-- Number of Susceptible
R0 = (g/b)S
When R0>1 then Pathogen
When R0<1 then Pathogen no
V. Observation: In many organisms, higher population growth
rate leads to higher fitness
Question: Why would diseases not evolve high virulence?
Observation: diseases are dependent on the host for their
Question: How do diseases modify the behaviors or physical
characteristics of hosts to promote transmission?
Behaviors or modifications induced in host that cause
transmission (see Figure 15.17)
VI. The ecology and evolution of AIDS
A. How does HIV work?
CD4 cells, or helper T cells, that initiate the immune response
-- Does not kill host
directly, but by weakening immune system
-- Does not kill
host quickly – usually within 15 years of infection
B. Evolution of HIV
1. Retroviruses prone to
error in transcription -- 1 every 1000 base pairs (107 times
typical mutation rates in humans)
2. Mutations provide
variability within hosts
3. Drugs provide a
4. Some virions are
better able to survive and reproduce
5. Large numbers,
fast population growth rate, and high mutation rate lead to rapid
F. Why is HIV fatal?
1. Assumption 1:
More virulent strains have higher population sizes and kill host
2. Assumption 2: More
virulent strains are more likely to be transmitted
-- transmission rate related to virulence
-- More virulent strains transmit faster,
but die sooner
-- Less virulent strains transmit slower,
but die later