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Third stage
Medicine
Lec-1
د
.
جاسم
محمد
1/1/2014
INTRODUCTION TO INFECTIOUS DISEASES
• Infection and the disease that results from it remain the greatest killer of human
• The concept of an infectious agent emerged in the mid-19th century and was defined
by a German physician, Robert Koch (1843-1910), When he isolated Bacillus
anthracis,(Kocks postultes).
The main characters of infectious diseases:
• 1. They are caused by living organism.
• 2. Communicability: Can be transmitted from human to human or from animals to
human (zoonosis).
• 3. Cure can occur either spontaneously or by giving antibiotics (except for some
viruses).
• 4. Many of them are preventable through different methods of prevention including
vaccination.
• 5. Permanent immunity can be resulted in some of them after exposure to the disease.
The causative agents of infection in man:
• Prions: These are the simplest infectious agents, and consist of an altered form of
normally occurring protein. They are causing a group of diseases in human being called
spongiform encephalopathy like Creutzfeldt-Jakob disease (CJD).
• Viruses: Viruses contain two types of macromolecule: proteins and nucleic acids,
(either DNA or RNA). They cannot reproduce by theirselves, but needing to enter a cell.
• Bacteria: They contain both RNA and DNA, have facilities for protein metabolism and
are generally free living, although some (e.g. Chlamydia and rickettsiae) are
intracellular parasites. Bacteria have no nuclei but reproduce autonomously. They
include mycoplasmas, spirochaetes, Actinomyces, bacilli and cocci.
• Others: include fungi ( e. g.candidia), protozoa (e. g. malaria, amoebiasis) and parasitic
worms like ascars, scistosoma (helminths).
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NFLUENCES ON PATTERNS OF INFECTION IN DEVELOPED COUNTRIES
• 1. Vaccines:
• 2. Animal husbandry and preparation of food: Salmonella originating in poultry and
eggs.
• 3. Microbial resistance: Staphylococcus aureus (meticillin-resistant, MRSA.
• 4. Sexual behaviour: Increase in HIV infection and other sexually transmitted diseases
• 5. International travel: Importation of malaria
• 6. Resurgence of infections: Tuberculosis-world-wide, especially in association with HIV
infection,
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• 7. Immunosuppression
• 8. Injection drug addiction
• 9. 'New' and emerging infections:Avian/pandemic 'flu
Infections in developing countries
• infection remains one of the most common causes of disease and death, particularly in
children
PATTERNS OF INFECTION IN DEVELOPING COUNTRIES
1. Killers of children, preventable but variably prevalent
• (Measles, Diphtheria, Pertussis, Poliomyelitis, Tetanus, Hepatitis B, Gastroenteritis,
Malaria, Meningococcal diseas, Acute diarrhoeal illness).
2. Chronic disabling infections, widely prevalent:
• (Leprosy, Tuberculosis, Trachoma, Malaria, Trypanosomiasis cruzi, Amoebiasis,
• Intestinal helminthes, Schistosomiasis, Filarial infection
3. Epidemic diseases, actual and potential
• (Louse-borne typhus and relapsing fever, Cholera, Malaria, Visceral leishmaniasis
• HIV infection, Tuberculosis in association with HIV epidemic, Influenza, Enteric fevers
4. Infections liable to focal outbreaks (zoonotic or vector-borne)
• Dengue fever, Cutaneous leishmaniasis, African trypanosomiasis, Plague, Anthrax,
• And Yellow fever).
FACTORS AFFECTING THE PATTERN OF INFECTIOUS DISEASES IN DEVELOPING
COUNTRIES
1. Widespread poverty:
• most developing countries cannot buy vaccines and drugs inaddition to malnutrition,
which makes people more susceptible to disease,.Vector control (e.g. malaria, African
trypanosomiasis) remains imperfect
2. Poor sanitation
• e. g. cholera spread, in which the infecting agent is transmitted through
contaminated sewage
3. Climate
• Indirectly reducing agricultural production, which increases the risk of malnutrition.
• hot weather and humid forests favor growth of the flies and mosquitoes that
transmit malaria, yellow fever, ……..
4. Finally, in developing countries (including tropical areas), infectious diseases are often
associated with natural disasters such as drought, flooding and earthquakes as well as with
war and political strife.
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MO and Host interaction
• Viruses, bacteria, fungi and protozoa all interact with the human body and do so in a
relationship that is
• - Symbiotic
• - Saprophytic
• - Parasitic
Symbiotic: The symbiotic e. g. colonising bacteria in the human gut, upper respiratory tract
and lower genital tract, and on the skin and superficial mucous membranes . Often the
human body benefits from this type of colonisation, as in the prevention of supervening
infection with Clostridium difficile by normal gut flora or Candida albicans by vaginal
lactobacilli.
Saprophytic: e.g.fungal skin infections with minimal local damage and little or no systemic
upset.
Parasitic: True infection only occurs when microorganisms upset normal physiology or
breach skin and/or mucous membranes and enter the blood stream or normally sterile
environments such as connective tissue, bones, muscles and joints or the central nervous
system . Subsequent illness is produced in the individual by the interaction of the host
immune mechanisms and the infecting organism and its toxic effects