Basics of infectious diseases
Lecture 1: introduction to infectious diseases
Infectious disease (ID): caused by pathogenic microorganisms, such as bacteria, viruses,
worm parasites, protozoa, or fungi
Communicable infectious disease: can be transmitted directly or indirectly, from one host to
another
Non-communicable infectious diseases: caused by opportunistic pathogen from an
individual’s own microflora
Emerging infectious diseases (EID): appears for the first time, or may have existed previously,
but is rapidly increasing in incidence or geographic range.
Infection: pathogen / parasite invades host
Disease: damaged tissue and function is impaired
Pathogen: microorganism capable of causing disease (-> = damage)
- Bacteria, fungi and viruses
Parasite: lives in another species (host) 0> sometimes associated with damage
- Metazoan (worms/insects) or protozoa (animal-like cells)
Determine if outbreaks is ID:
Symptoms: fever, diarrhea, fatigue, coughing, muscle pain
o Diagnosis (or not: maybe toxin but no ID)
o -> identify and count cases: person / place / time
Common cause OR individual cases?
Continue outbreak investigation:
1. What is the pathogen?
2. What is the source?
a. One = common source epidemic
b. Multiple = propagating epidemic
3. Transmission?
a. Human-human
b. Or zoonosis: vector / intermediate host(s) / reservoir
Control ID outbreak:
Isolate / cure / break transmission cycle
Outbreak control:
- Prevention of exposure: containment of source and break transmission cycle
- Prevention of infection: protection with vaccination, safe water, sanitation and
adequate shelter
- Prevention of disease: prophylactic treatment of high-risk group
Zoonoses: disease / infection transmissible from animals to humans
Host types/ role:
Definitive host: host in which pathogen or parasite reach maturity and reproduces
Intermediate host: pathogen undergoes an essential developmental transition
Transportation host: pathogen undergoes no essential developmental transition
Reservoir: long-term natural host of a pathogen that often does not cause a disease in this
host.
1
,Endemic: commonly / constantly present in population ( low level)
Epidemic: sudden increase in particular population
Pandemic: geographically widespread population
Main types epidemics -> (key feature = transmission)
1. Human to human -> propagated epidemics
2. No human to human -> common source epidemics
Latent period: time interval from initial exposure (*) until start of transmission to another
host
Incubation period: time interval from initial infection until onset of clinical disease (typically
has a range)
Period of communicability / infectious period: period during which a person is infectious and
enabling transmission
2
,Trade-off in host pathogen interactions:
Virulence vs. transmission rate
A low level of reproduction of a pathogen / parasite has little impact on host fitness,
but also results in little transmission, whereas a high level of reproduction yields high
transmission, but only during the short lifetime of the diseased host.
The optimal virulence of the parasite is achieved by balancing virulence such that its
transmission success (reproductive rate) is maximized over lifetime of infection.
Basic reproductive rate (R0)
Number of secondary infections by a case of an infection during its entire transmission
lifetime when a population is totally susceptible transmission success
R0 = c (number of contacts) * I (infectiousness) * D (duration)
Epidemic dies out, R0 <1
Herd immunity: resistance of host population to pathogen transmission because of immunity
of a large percentage of the population.
Exploits the effective reproduction rate (R) of infectious disease agents
R= R0 * % of susceptible individuals in a population
Herd immunity threshold (HIT (R0-1)/R0 ) = the % of susceptible population that needs to
be protected by vaccination to stop an epidemic outbreak
Bv. R0 = 4 (4-1)/4 = 0.75 -> 75% of population must be vaccinated
BMR vaccine: mumps, measles, rubella
Prevalence: total number of cases
Incidence: # new cases within population during time period
Morbidity: ‘’how ill”
Mortality: ‘’ how deadly’’
Morbidity / mortality rate: how many ill / how many killed
DALY (disability-adjusted life years): counts years of ‘healthy’ life lost due to poor health or
disability (YLD) plus years of life lost due to premature death (YLL)
DALY = YLD + YLL
YLD = I * DW * L
YLL = N * L
I= cases #
L = duration until remission or death
DW = disability weight
N = # of deaths
L = standard life expectancy at age of death in years
DALY = counts equivalent years of ‘healthy’ life lost due to poor health or disability and
potential years of life lost due to premature death
3
, Emerging of ID caused by alterations in pathogens in hosts, and / or in the environment
Gain of virulence by pathogen:
- Antibiotic resistance (MRSA or superbug)
- Anti-retroviral drug resistance (HIV)
- Antigenic drift influenza (by mutations)
- Antigenic shift from animals to humans (by rearrangement)
Climate change:
- Vector-borne infections
- Food-borne infections
- Water-borne infections
also tropical disease to Europe
Enhanced transmission
- Changes in human demographics (pop. Growth, urbanization, migration, mobility)
- Poverty and social inequality (poverty, hygiene, contaminated food and water,
inadequate governance/investments, food security, lack of public health service)
- Technology and industry (agriculture development, globalization of food supply)
- Human susceptibility to infections (opportunistic re-emerging infections in >1% of
world population which are immune-compromised: aids/HIV, immune diseases,
chemo, transplants, pregnancy)
Lecture 2: biology of bacteria
The cell wall:
Gram positive: thick peptidoglycan layer and no outer lipid membrane -> purple
Gram negative: thin peptidoglycan layer and have an outer lipid membrane -> ‘pink’
Cholera
- Diarrheal disease
- Caused by: vibrio cholerae
- Very rapid disease progression
- Food and water borne
- Chloride channel activated -> raises lumen osmotic pressure
Black death / bubonic plague / great plague
- Spread by fleas, 75 – 100 M killed, 30 % of population
4
Lecture 1: introduction to infectious diseases
Infectious disease (ID): caused by pathogenic microorganisms, such as bacteria, viruses,
worm parasites, protozoa, or fungi
Communicable infectious disease: can be transmitted directly or indirectly, from one host to
another
Non-communicable infectious diseases: caused by opportunistic pathogen from an
individual’s own microflora
Emerging infectious diseases (EID): appears for the first time, or may have existed previously,
but is rapidly increasing in incidence or geographic range.
Infection: pathogen / parasite invades host
Disease: damaged tissue and function is impaired
Pathogen: microorganism capable of causing disease (-> = damage)
- Bacteria, fungi and viruses
Parasite: lives in another species (host) 0> sometimes associated with damage
- Metazoan (worms/insects) or protozoa (animal-like cells)
Determine if outbreaks is ID:
Symptoms: fever, diarrhea, fatigue, coughing, muscle pain
o Diagnosis (or not: maybe toxin but no ID)
o -> identify and count cases: person / place / time
Common cause OR individual cases?
Continue outbreak investigation:
1. What is the pathogen?
2. What is the source?
a. One = common source epidemic
b. Multiple = propagating epidemic
3. Transmission?
a. Human-human
b. Or zoonosis: vector / intermediate host(s) / reservoir
Control ID outbreak:
Isolate / cure / break transmission cycle
Outbreak control:
- Prevention of exposure: containment of source and break transmission cycle
- Prevention of infection: protection with vaccination, safe water, sanitation and
adequate shelter
- Prevention of disease: prophylactic treatment of high-risk group
Zoonoses: disease / infection transmissible from animals to humans
Host types/ role:
Definitive host: host in which pathogen or parasite reach maturity and reproduces
Intermediate host: pathogen undergoes an essential developmental transition
Transportation host: pathogen undergoes no essential developmental transition
Reservoir: long-term natural host of a pathogen that often does not cause a disease in this
host.
1
,Endemic: commonly / constantly present in population ( low level)
Epidemic: sudden increase in particular population
Pandemic: geographically widespread population
Main types epidemics -> (key feature = transmission)
1. Human to human -> propagated epidemics
2. No human to human -> common source epidemics
Latent period: time interval from initial exposure (*) until start of transmission to another
host
Incubation period: time interval from initial infection until onset of clinical disease (typically
has a range)
Period of communicability / infectious period: period during which a person is infectious and
enabling transmission
2
,Trade-off in host pathogen interactions:
Virulence vs. transmission rate
A low level of reproduction of a pathogen / parasite has little impact on host fitness,
but also results in little transmission, whereas a high level of reproduction yields high
transmission, but only during the short lifetime of the diseased host.
The optimal virulence of the parasite is achieved by balancing virulence such that its
transmission success (reproductive rate) is maximized over lifetime of infection.
Basic reproductive rate (R0)
Number of secondary infections by a case of an infection during its entire transmission
lifetime when a population is totally susceptible transmission success
R0 = c (number of contacts) * I (infectiousness) * D (duration)
Epidemic dies out, R0 <1
Herd immunity: resistance of host population to pathogen transmission because of immunity
of a large percentage of the population.
Exploits the effective reproduction rate (R) of infectious disease agents
R= R0 * % of susceptible individuals in a population
Herd immunity threshold (HIT (R0-1)/R0 ) = the % of susceptible population that needs to
be protected by vaccination to stop an epidemic outbreak
Bv. R0 = 4 (4-1)/4 = 0.75 -> 75% of population must be vaccinated
BMR vaccine: mumps, measles, rubella
Prevalence: total number of cases
Incidence: # new cases within population during time period
Morbidity: ‘’how ill”
Mortality: ‘’ how deadly’’
Morbidity / mortality rate: how many ill / how many killed
DALY (disability-adjusted life years): counts years of ‘healthy’ life lost due to poor health or
disability (YLD) plus years of life lost due to premature death (YLL)
DALY = YLD + YLL
YLD = I * DW * L
YLL = N * L
I= cases #
L = duration until remission or death
DW = disability weight
N = # of deaths
L = standard life expectancy at age of death in years
DALY = counts equivalent years of ‘healthy’ life lost due to poor health or disability and
potential years of life lost due to premature death
3
, Emerging of ID caused by alterations in pathogens in hosts, and / or in the environment
Gain of virulence by pathogen:
- Antibiotic resistance (MRSA or superbug)
- Anti-retroviral drug resistance (HIV)
- Antigenic drift influenza (by mutations)
- Antigenic shift from animals to humans (by rearrangement)
Climate change:
- Vector-borne infections
- Food-borne infections
- Water-borne infections
also tropical disease to Europe
Enhanced transmission
- Changes in human demographics (pop. Growth, urbanization, migration, mobility)
- Poverty and social inequality (poverty, hygiene, contaminated food and water,
inadequate governance/investments, food security, lack of public health service)
- Technology and industry (agriculture development, globalization of food supply)
- Human susceptibility to infections (opportunistic re-emerging infections in >1% of
world population which are immune-compromised: aids/HIV, immune diseases,
chemo, transplants, pregnancy)
Lecture 2: biology of bacteria
The cell wall:
Gram positive: thick peptidoglycan layer and no outer lipid membrane -> purple
Gram negative: thin peptidoglycan layer and have an outer lipid membrane -> ‘pink’
Cholera
- Diarrheal disease
- Caused by: vibrio cholerae
- Very rapid disease progression
- Food and water borne
- Chloride channel activated -> raises lumen osmotic pressure
Black death / bubonic plague / great plague
- Spread by fleas, 75 – 100 M killed, 30 % of population
4
