Unit 6 Infectious diseases & immunity - shortend.pdf

Infectious diseases are diseases, Which has an ability of transferring from obe person to another person

1. General Biology
Unit 6
Infectious diseases and Immunity





12/4/2023 1

2. Principles of infectious diseases
Infectious diseases are diseases caused by infectious agents
– bacteria, viruses, fungi, protozoa, helminthes & prions.
It occurs as the result of interactions b/n pathogenic
microorganisms & the host.

All infectious diseases begin at some surface of the host,
–external surfaces e.g; skin
–internal surfaces e.g; mucous membranes of the respiratory
tract, intestine, urogenital tract.
12/4/2023 2

3. To cause infectious disease, pathogen must:
 enter the host
 establish-metabolize & multiply on or in host
 resist host defenses
 damage the host

Entrance to the host may occur through entry routes.
e.g, mouth, eyes, genital openings, wounds, etc
Pathogens multiply & grow with in the host body
pathogens may cause tissue damage in some cases.

12/4/2023 3
Principles of infectious diseases …

4. Important term
Infection results when pathogen enters & begins growing within a
host.
Disease occurs when the host body are damaged, tissue function is
impaired & signs & symptoms of an illness appear.
Symptom: a change in body function that is felt by a patient as a
result of disease.
Sign: a change in a body that can be measured or observed as a
result of disease.
Syndrome: a specific group of signs & symptoms that accompany a
disease
12/4/2023 4

5. Classifying infectious diseases by occurrence of diseases
Incidence: fraction of a population that contracts a disease during a
specific time
Prevalence: fraction of a population having a specific disease at a
given time
Sporadic disease: disease that occurs occasionally in a population
Endemic disease: disease constantly present in a population
Epidemic disease: disease acquired by many hosts in a given area in
a short time
Pandemic disease: worldwide epidemic

12/4/2023 5
Important terms…

6. Scope of infectious disease can be defined as:-
Acute: disease develops rapidly
Chronic: disease develops slowly
Subacute: symptoms appear between acute & chronic
Latent: disease with period of no symptoms when the causative agent is inactive.
 Disease development and stages
Incubation period: time interval b/n infection & appearance of signs
Prodromal period: characterized by appearance of first mild signs & symptoms.
Period of illness: disease at its height: all disease signs & symptoms apparent.
Period of decline: signs & symptoms subside.
Period of convalescence: body returns to pre diseased state, health is restored.

12/4/2023 6
Important terms…

7. Fig: 6.1 development and stages of a disease
12/4/2023 7

8. An infection of infectious disease can be
Local infection: infection limited to a small area of the body
e.g; abscesses
Systemic infection: an infection throughout the body by blood
and/or lymph
–Bacteremia: bacteria in the blood.
–Sepsis: toxic inflammatory condition arising from the spread of
microbes (bacteria or their toxins) from a focus of infection
–Septicemia: growth of bacteria in the blood.
–Toxemia: toxins in the blood.
–Viremia: viruses in the blood.
Focal infection: systemic infection that began as a local infection

12/4/2023 8
Important terms…

9. Primary: is an acute infection that causes initial illness.
Secondary: occurs after host is weakened from primary infection
Subclinical (inapparent): no noticeable signs and symptoms

Classifying infectious diseases
Communicable disease: a disease that is spread from one host to
another
Contagious disease: a disease that is easily spread from one host to
another
Non-communicable disease: a disease that is not transmitted from
one host to another


12/4/2023 9
Important terms…

10. Pathogenicity
We contact numerous microorganisms daily.
Most microbes are harmless & many are beneficial.
Few are pathogens that cause diseases.
These microorganisms are classified based on their pathogenicity:-
Primary pathogen is microbe or virus that causes disease in
healthy individual.
Diseases such as plague, malaria, measles, influenza, diphtheria,
tetanus, TB, etc.
These pathogenic organisms are called virulent because of their
virulence (degree of pathogenicity) factor.
12/4/2023 10

11. Opportunistic pathogen causes disease when body’s immunity
(innate or adaptive defenses) are compromised.
•Factors for susceptibility include:
–malnutrition
–genetic defects,
–AIDS & other disease
–Cancer
–alcohol or drug abuse
–immunosuppressive therapy


12/4/2023 11

12. Infectious disease and their causative agent
The common pathogens known as causative agents for different types
of infectious diseases are:-
Bacteria:
•are unicellular prokaryotic organisms;
•Morphologically: bacillus (rod-shaped),coccus (spherical), spirillum (helical rods).
•are frequently divided into two broad classes based on Gram stain reaction.
•Gram-negative bacteria: Salmonella typhi, Yersinia pestis
•Gram-positive bacteria: Staphylococcus aureus, Clostridium tetani
Fungi:
•Many fungal infections will appear in the upper layers of the skin, and some
progress to the deeper layers.
•Those with a higher risk of developing a fungal infection include people who:
–use strong antibiotics for a long time
–have a weakened immune system,
•Ringworm and athlete's foot, candidiasis, histoplasmosis


12/4/2023 12

13. Infectious Disease and their Causative Agent …
Viruses
•Viruses can infect all organisms.
•Viruses are not organisms themselves
–they have no metabolism and cannot reproduce.
•A virus particle is composed of a viral genome of nucleic acid that is surrounded by a
protein coat called a capsid.
•Viral genomes may be DNA (DNA virus), RNA (RNA virus).
•Viruses cause disease by disrupting normal cell function.
–repressor proteins that stop the synthesis of proteins, RNA, and DNA.
–weaken cell membranes and lysosomal membranes, leading to cell autolysis.
–toxic to cells, immune defenses also may kill virus-infected cells.
Prions
•proteins that contains no genetic material.
•When the protein folds into an abnormal shape, it can become a rogue agent
–Creutzfeldt-Jakob disease (in humans), scrapie (in sheep), and bovine spongiform
encephalopathy ("mad cow disease" in cattle);

14. Infectious Disease and their Causative Agent …
Protozoa
•are unicellular, heterotrophic eukaryotes.
•Protozoal infections can be acquired through:
–contaminated food or water, e.g. Giardia lamblia & Cryptosporidium
parvum
–bite of an infected arthropod such as a mosquito, e.g. Plasmodium
species that cause Malaria
Helminths
•are simple, invertebrate animals, some of which are infectious parasites.
•are multicellular and have differentiated tissues.
•Because they are animals, their physiology is similar in some ways to ours.
•Many helminths have complex reproductive cycles that include multiple
stages, many or all of which require a host.
•The common helminthes are Ascaries, tape worm, hook worm, Schistosoma

15. Modes of transmission
•Transmitted through either direct or indirect contact.
Direct contact occurs when an individual is infected by contact with
the reservoir, for example,
–touching an infected person
–ingesting infected meat, or
–being bitten by an infected animal or insect.
–inhaling the infectious agent in droplets emitted by sneezing or
coughing
–intimate sexual contact.
e.g:-ringworm, AIDS, trichinosis, influenza, rabies, and malaria.
12/4/2023 15

16. Indirect contact occurs when a pathogen can withstand the
environment outside its host for a long period of time before
infecting another individual.
–inanimate objects that are contaminated
–ingesting food & beverages contaminated
–fecal-oral route of transmission, in which sewage-contaminated
water for drinking, washing, or preparing foods,
e.g:- cholera, rotavirus infection, cryptosporidiosis, and giardiasis.
•These modes of transmission are all examples of horizontal
transmission because the infectious agent is passed from person to
person in a group.
•Some diseases also are transmitted vertically- Congenital transmission

12/4/2023 16

17. Modes of transmission
Table: Survival times of microorganisms on hard inanimate surfaces
Pathogens Survival Time
•Adenovirus Up to 3 months
•Clostridium difficile Up to 5 months
•Coronovirus 3 hours
•E. coli Up to 16 months
•Influenza 1-2 days
•MRSA Up to 7 months
•M. tuberculosis Up to 4 months
•Norovirus Up to 7 days


BMC Infectious Diseases 2006: 6: 130

18. Host defenses against infectious diseases
Overview of the immune system
•The immune system is the body’s defense mechanism against foreign invaders.
•Edward Jenner, pioneered method of vaccination/immunization in /1796/.
•Louis Pasteur established protocols:
–to identify the cause of infectious diseases
–the medical basis for immunization.
•formulated his germ theory of disease - disease is caused by microorganisms.
•Observed conserved aged cultures of fowl cholera lost its power to induce disease in
chickens
–still conferred immunity to the disease when injected.
–attenuated (weakened) cultures of anthrax and rabies to vaccinate against those
diseases.

19. Host defenses against infectious diseases
Overview of the immune system
•The immune system is composed of different cell types, tissues & organs.
•Many of these cells are organized into separate lymphoid organs or glands.
•The immune system has a mobile force of cells in the bloodstream
–ready to attack the invading microbe.
•They maintain communication through cell contact & molecules secreted by them
•So, immune system has been linked to the nervous system.
The human body has several general mechanisms for preventing infectious
diseases.
–nonspecific or innate defenses mechanism
(operate against a wide range of pathogens)
–specific or adaptive defenses mechanism
(they target particular pathogens & pathogen-infected cells).

20. Nonspecific mechanisms (Innate Immune system)
The first line of defense which are present at birth.
•Anatomical barriers:
–skull & vertebral column, skin, mucous membrane
•Physiological deterrents:
–Acidic skin secretions, sweat, saliva, tears, vaginal secretions, clotting
factors of blood plasma,
•Phagocytosis:
–Macrophages, monocytes, neutrophils, dendritic cells,
•Inflammation:
–Inflammatory cytokines, complements
–Involve swelling, reddening, elevated temperature, and pain
–May cause tissue damage
•Normal flora:
–Non-pathogenic microorganisms normally leave in body
–“Opportunistic” agents may be able to infect and cause disease

21. Specific mechanisms (Adaptive defense)
•If nonspecific mechanisms fail, the body initiates a second, specific line of
defense.
•Adaptive system shows specificity & immunological memory.
Enables to target particular pathogens and pathogen infected cells for
destruction
Leads to a rapid removal of pathogens on next time of entry.
•Adaptive system depends on specialized WBCs called lymphocytes &
includes:
T-cells – produced from lymphocytes that matured in thymus gland
B-cells – produced from lymphocytes that matured in bone marrow

22. Specific mechanisms (Adaptive defense)
•The two complementary specific immune response are cell-mediated
(CM) and antibody-mediated (AM) responses.
•CM response involves T-cells responsible for
–Directly destroying body cells that are infected with a virus or cancerous cells,
–activating other immune cells to be more efficient microbe killers.
•AM response involves both T-cells & B-cells
–destruction of invading pathogens as well as the elimination of toxins.
•Macrophages are antigen presenting cells
–engulf & digest pathogens & display pathogens’ antigens on their surface
•This display stimulate helper T (TH)-cells to release signal molecules called
lymphokines.
•lymphokines stimulate CM & AM responses.

23. Specific mechanisms (Adaptive defense)
The branches of acquired immunity
I. Humoral or AM immunity
–Consists of antibodies circulating in the fluids of the body
–Targets are mainly extracellular microbes and parasites
–Produced by B-lymphocytes
II. Cellular or CM immunity
–Mediated by lymphocytes (T-cells) themselves
–Targets – virus or parasite-infected cells, cancer cells, foreign graft cells
–Can act directly by lysing foreign cells or indirectly by releasing chemicals

24. overview of cell-mediated and antibody-mediated immune responses

25. Summary of the Immune Response
Fig. 6.3 Patterns and layers of responses against pathogens

26. Adverse immune reactions (responses)
•The aim of the immune responses is to protect the body
•Adverse immunological reactions can occur due to:
–Exaggerated immune response
–producing inappropriate immune response against self components.
–failure of appropriate recognition mechanism.
•This can lead to adverse reactions (immunopathology).
•This kind of immune disorder can be divided as:
–Hypersensitivity
–Autoimmune diseases
–Immunodeficiency

27. 12/4/2023 27

28. Fig. 6.5. Types of hypersensitivity reactions (source, Kuby immunology)
12/4/2023 28