Download Nature of Infectious Diseases

Nature of Infectious
Diseases
Understanding Infectious Diseases
 Disease: the term refers to conditions that
impair normal tissue function.




Cystic fibrosis
Atherosclerosis
Measles
All are diseases but there are fundamentally
different causes for each disease
 Infectious disease: a disease caused by the
invasion of a host by agents whose activities
harm the host’s tissues and can be
transmitted to others (they are infectious)
 Pathogens: microorganisms that are capable
of causing disease. A true pathogen is one
that causes disease in virtually every
susceptible host.
 The terms “infections” and “disease” are not
synonymous.
 An infection results when a pathogen
invades and begin growing in the host
 A disease results only if tissue function is
impaired by the invasion and growth of a
pathogen.


polio virus
Ebola hemorrhagic fever
 Virulent: degree or intensity of pathogenicity
of an organism. It is indicated by mortality
rate from the related disease and/or ability to
invade tissue and cause disease.
Infectious Disease Process
 In order for a pathogen to cause disease they
must be able to:




Enter the host body
Adhere to specific host cells
Invade and colonize host tissues
Inflict damage on host tissue
 Entry to the host is usually through a natural
orifice (mouth, eyes, genitals, wounds)
 Attach to specific cells to invade other body
tissues
 Some pathogens inflict damage by growing;
others produce destructive toxins
Modes of Transmission
 May be transmitted through direct or indirect contact
 Direct contact: occurs when an individual is infected
by contact with the reservoir
 Indirect contact: occcurs when a pathogen can
withstand the environment outside its host for a long
period of time before infecting another individual
 Horizontal transmission: agent is passed from person
to person
 Vertical transmission: agent is transmitted from mother
to baby
 Disease reservoir: the environment where the
infectious agent survives



Examples: humans or animals
Clostridium tetani disease reservoir is the soil
Yersenia Pestis is rodents
Microbes that cause infectious diseases
 Bacteria: unicellular prokaryotic organism (no
organized internal membranous structure)
that reproduce by growing and dividing into
two cells
a. bacillus (rod-shaped)
b. coccus (spherical)
c. spirillum (helical rods)
 Virus: infectious agent composed of a protein
coat and a single type of nucleic acid. Lacks
an independent metabolism and reproduces
only within a host cell
 Fungi: eukaryotic with rigid cellulose walls
and reproduce primarily by forming spores
 Protozoa: unicellular eukaryotes capable of a
variety of rapid and flexible movements
 Helminthes: simple, invertebrate animals that
are infectious parasites. Drugs that kill
helminthes are frequently toxic to humans.
 Prions: infectious particles that consist only
of proteins. Infections usually result in brain
tissue that is riddled with holes.