Lecture 1 The Global Impact of
Infectious Diseases
Viruses, bacteria, and fungi often cause acute infections.
Parasites – protozoa (protists) and worms (helminths)
Symbiosis – An interaction between two different organisms
living in close physical association (living together.)
Parasitism – a relationship between two species in which one species benefit
and the other species s harmed
Stages of infectious disease
Lecture 1 The Global Impact of Infectious Diseases 1
, Incubation period – time between infection and the occurrence of first
symptoms or signs of disease
Prodromal period – short time of generalised, mild symptoms (such as fatigue)
– not all infectious diseases have this stage.
Illness – most severe stage when symptoms are most evident, and host immune
system not yet fully responded.
Convalescence – body gradually returns to normal (variable time depending on
pathogen and damage)
Biological Response Gradient
Clinical picture depends on:
Infecting dose
Age of host
Biological sex
Host genetics
Host nutritional status
Co-infection with other pathogens
The progression of disease
Invasion
Inhalation - Influenza (virus), SARS-CoV-2 (virus), Yersinia pestis (bacterium)
Oral Transmission – Ascaris (nematode), Taenia Solium (cestode)
Intra-uterine – toxoplasma (protist)
Sexual transmission – HIV (virus)
Direst Skin contact – Hookworm (nematode), schistosome (trematode)
Direct inoculation – Injections, HIV (virus)
Vector borne disease – Plasmodium (protist), yersinia pestis (bacterium),
Wuchereia bancrofti (nematode)
Multiplication
Protists - can cause disease following inoculation of only a few infectious stages
as they can multiply within the body - disease severity may depend on how
Lecture 1 The Global Impact of Infectious Diseases 2
, quickly they multiply.
Helminths – most helminths cannot multiply within the body and so disease
severity is dependent upon the number of infectious stages acquired by host
over time
Spread
The ability of the organism to move from the initial site of infection to infect other
areas of the body.
Movement between body systems
Some infectious agents undergo developmental changes at the same time, this
can have implications for the host immunity response.
Pathogenesis
Causation and development of clinical disease, influenced by the no. of
pathogenic organisms present, the virulence of the organism, and the reaction of
the host (degree of resistance.)
Virulence of the organisms e.g. Direct killing of host cells e.g. Plasmodium
(malaria); Blockages within host organs e.g. Plasmodium (malaria); Toxins e.g.
bacterium, Yersinia pestis (plague); Inappropriate activity of host immune system
–
immunopathology e.g., schistosomes
Global burden of disease
Incidence - number of new cases of infection occurring in a population in a
defined period of time.
Prevalence - total number of infected individuals in a population at a given
point in time i.e. the number of new and old cases
Mortality - total number of deaths from disease in a population in a defined
period of time
In some countries there has been a shift in cause of death from infectious
disease to things like cancer, and heart disease.
Lecture 1 The Global Impact of Infectious Diseases 3
, Map showing adult mortality rate of both sexes (2016), more of a burden from infectious diseases in
some countries compared to others.
High-income countries
70% of deaths - people aged 70 years and older.
Only 1% of deaths among children under 15 years
Predominantly die of chronic diseases: cardiovascular diseases, cancers,
dementia, chronic obstructive lung disease or diabetes
Low-income countries
40% of deaths - children under 15 years
20% deaths - people aged 70 years and older.
Predominantly die of infectious diseases: respiratory infections, HIV/AIDS,
diarrhoeal diseases, malaria and tuberculosis - collectively account for 1/3 of
deaths in low-income countries
Measuring the burden of disease
Morbidity: ill health associated with disease
DALY (disability adjusted life year)
Measures ‘life years’ lost due to premature mortality and equivalent years
lost because of morbidity (i.e., lower 'quality of life ‘)
1 DALY - calculated by adding together 'years of life lost (YLL) to premature
mortality' and 'years lost to (lived with) disability (YLD)‘for people living with
Lecture 1 The Global Impact of Infectious Diseases 4
Infectious Diseases
Viruses, bacteria, and fungi often cause acute infections.
Parasites – protozoa (protists) and worms (helminths)
Symbiosis – An interaction between two different organisms
living in close physical association (living together.)
Parasitism – a relationship between two species in which one species benefit
and the other species s harmed
Stages of infectious disease
Lecture 1 The Global Impact of Infectious Diseases 1
, Incubation period – time between infection and the occurrence of first
symptoms or signs of disease
Prodromal period – short time of generalised, mild symptoms (such as fatigue)
– not all infectious diseases have this stage.
Illness – most severe stage when symptoms are most evident, and host immune
system not yet fully responded.
Convalescence – body gradually returns to normal (variable time depending on
pathogen and damage)
Biological Response Gradient
Clinical picture depends on:
Infecting dose
Age of host
Biological sex
Host genetics
Host nutritional status
Co-infection with other pathogens
The progression of disease
Invasion
Inhalation - Influenza (virus), SARS-CoV-2 (virus), Yersinia pestis (bacterium)
Oral Transmission – Ascaris (nematode), Taenia Solium (cestode)
Intra-uterine – toxoplasma (protist)
Sexual transmission – HIV (virus)
Direst Skin contact – Hookworm (nematode), schistosome (trematode)
Direct inoculation – Injections, HIV (virus)
Vector borne disease – Plasmodium (protist), yersinia pestis (bacterium),
Wuchereia bancrofti (nematode)
Multiplication
Protists - can cause disease following inoculation of only a few infectious stages
as they can multiply within the body - disease severity may depend on how
Lecture 1 The Global Impact of Infectious Diseases 2
, quickly they multiply.
Helminths – most helminths cannot multiply within the body and so disease
severity is dependent upon the number of infectious stages acquired by host
over time
Spread
The ability of the organism to move from the initial site of infection to infect other
areas of the body.
Movement between body systems
Some infectious agents undergo developmental changes at the same time, this
can have implications for the host immunity response.
Pathogenesis
Causation and development of clinical disease, influenced by the no. of
pathogenic organisms present, the virulence of the organism, and the reaction of
the host (degree of resistance.)
Virulence of the organisms e.g. Direct killing of host cells e.g. Plasmodium
(malaria); Blockages within host organs e.g. Plasmodium (malaria); Toxins e.g.
bacterium, Yersinia pestis (plague); Inappropriate activity of host immune system
–
immunopathology e.g., schistosomes
Global burden of disease
Incidence - number of new cases of infection occurring in a population in a
defined period of time.
Prevalence - total number of infected individuals in a population at a given
point in time i.e. the number of new and old cases
Mortality - total number of deaths from disease in a population in a defined
period of time
In some countries there has been a shift in cause of death from infectious
disease to things like cancer, and heart disease.
Lecture 1 The Global Impact of Infectious Diseases 3
, Map showing adult mortality rate of both sexes (2016), more of a burden from infectious diseases in
some countries compared to others.
High-income countries
70% of deaths - people aged 70 years and older.
Only 1% of deaths among children under 15 years
Predominantly die of chronic diseases: cardiovascular diseases, cancers,
dementia, chronic obstructive lung disease or diabetes
Low-income countries
40% of deaths - children under 15 years
20% deaths - people aged 70 years and older.
Predominantly die of infectious diseases: respiratory infections, HIV/AIDS,
diarrhoeal diseases, malaria and tuberculosis - collectively account for 1/3 of
deaths in low-income countries
Measuring the burden of disease
Morbidity: ill health associated with disease
DALY (disability adjusted life year)
Measures ‘life years’ lost due to premature mortality and equivalent years
lost because of morbidity (i.e., lower 'quality of life ‘)
1 DALY - calculated by adding together 'years of life lost (YLL) to premature
mortality' and 'years lost to (lived with) disability (YLD)‘for people living with
Lecture 1 The Global Impact of Infectious Diseases 4
