Infection and Disease

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Relationships Between Hosts and Microorganisms:

Mutualistic: both partners benefit

i.e. gut microflora → bacteria receive warm nutrient rich habitat, synthesize vitamin K for the host

Commensal: neither host nor microbe are harmed

i.e microbial skin inhabitants

Parasitic: one partner benefits, the other is harmed

microbe based disease

Disease-Causing Microbes:

Pathogen: a disease-causing microbe → parasitic microbe

Pathogenicity: microbe's ability to enter a host and cause disease

Virulence: the degree of pathogenicity

depends on genetics of the species/strain (strain = breed) → strains can cross-breed (just like dogs)

some strains more virulent than others

passage of a pathogen through animals of the same species can make it more virulent

Invasiveness: ability of a pathogen to penetrate tissues and spread

successful invasiveness requires pathogens to have virulence factors

Virulence Factors: molecules produced by pathogens that contribute to the pathogenicity of the organism and enable them to colonize a niche in the host → pathogens must adapt to a new environment when they enter a host

Enzymes can help pathogens resist body defenses

Coagulase → staphylococci → forms blood clot that protects from phagocytosis

Streptokinase → dissolves fibrin clots → allows dissemination of bacteria

Hyaluronidase → enhances pathogen penetration through tissues

Leukocidins → disintegrate neutrophils and macrophages (cytotoxin)

Hemolysins → dissolve RBCs (cytotoxin)

virulence can be enhanced in biofilms because immune cells cannot reach bacterial cells

Pathogenicity Islands: gene clusters responsible for virulence

Transposal: passage of a human pathogen through a different animal

attenuates (weakens) the pathogen so it can be used as a vaccine

Parasitism: pathogens cause damage and disease in host

Toxigenicity: ability of a pathogen to produce toxins

Toxemia: presence of toxins in the blood

Exotoxins: proteins produced during bacterial metabolism → cytotoxins (kill cells), neurotoxins (act on nervous system), enterotoxins (act on digestive tract)

Toxoids: toxins whose toxicity has been destroyed but still elicit and immune response (structure might be picked up by immune system)

Endotoxins: released upon disintegration of gram-negative cells (lipopolysaccharides) → cause blood coagulation, fever → endotoxic shock may occur with antibiotic treatment of diseases caused by gram-negative bacilli

Types of Diseases and Infections:

Latrogenic Diseases*: caused by medical procedures

Idiopathic Diseases*: unknown causes

Nosocomial Infections: contracted in hospital, doctor's office, clinic, etc.

health-care associated infections (HAIs)

result of receiving treatment for another condition

host already compromised

caused by opportunistic agents

finding the Chain of Transmission key to reducing nosocomial infections (i.e. tracing back to root cause of infection)

* may or may not be infectious → latrogenic disease could be a result of surgical error (non-infectious), but could also be a result of doctor improperly washing hands (infectious)

Exogenous Infection: occurs if pathogen breaches host's external defense and enters sterile tissue

Endogenous Infection: normal microbiota enter sterile tissue

Opportunistic Infections: commensals take advantage of a change in the body's environment that favours the microbe

i.e. takes advantage of a change in the body/immune system

Primary Infections: occur in otherwise healthy bodies

Secondary Infections: occur in bodies weakened by primary infections

Local Diseases: restricted to a single area → i.e. boils

Systemic Diseases: disseminate to organs and systems

Communicable Diseases: contagious

Noncommunicable Diseases: not easily transmitted to another host

acquired directly from environment → i.e. tetanus

Septicemia: bacteria proliferate in the blood

Bacteremia/Viremia: pathogens transported by blood but do not multiply in it

The Microbiome:

Microbiome: the population of microbes that reside in the body without causing disease

begins at birth

a healthy fetus is sterile in the womb

by adulthood, the body contains 10 times more microbial cells than human cells

Mutualism: both host and microbe benefit

Commensalism: microbe benefits, host is unaffected

antibiotics can harm the microbiome

probiotics may restore microbiota after taking antibiotcs → i.e. lactobacilla (yogurt)

Transmission of Disease:

pathogens may enter humans through orifices such as the mouth, nose, genital openings, ears, eyes, and wounds in the skin (portals of entry)

inhalation

fecal-oral route

STDs

parenteral (through the skin)

Vectors: act as intermediates in the transmission of disease

living organisms

mechanical → transported on feet (i.e. flies)

biological → have life cycle in arthropod (i.e. mosquitoes)

Herd Immunity: general immunity to a pathogen in a population based on the acquired immunity of most members of the group over time

disease can be severe when transmitted to a population lacking herd immunity

i.e. native North Americans lacked herd immunity to smallpox and measles brought by Europeans

up to 90% of a population may be immune to a pathogen when herd immunity is present

Direct Contact: involves close personal contact with infected person

Horizontal transmission → transmitted among individuals of the same generation → i.e. kissing, respiratory droplets

Vertical transmission → transmission from mother to offspring

Indirect Contact: can involve non-living objects

fomites → i.e. hepatitis, athlete's foot

vehicle transmission through contaminated food/water → i.e. hepatitis, cholera

aerosols → small particles suspended on air currents → i.e. measles

vectors → i.e. malaria, Lyme disease

Process of Infection and Disease:

Pathogen Entry:

depends on cell adhesion and the infectious dose (# of microbes entering the body)

many pathogens have adhesins (i.e. capsules, flagella, pili) that adhere to specific tissues

may be able to attach to membrane proteins

capsules → protein/sugar coat surrounding bacteria → adhesive properties

pili → cause adherence/clumping → biofilms

flagella → may have adhesive properties

may use phagocytosis by body cells to enter cells or pass through defenses undetected

Virulence factors aid in pathogen entry (invasiveness)

Signs: characteristics of disease that can be observed by others

Symptoms: characteristics of disease only observed/felt by the patient

Syndrome: combination of signs and symptoms that occur together → indicative of a particular disease

Incubation Period: time between entry of the microbe and symptom appearance → little or no ill effect noted

Prodromal Phase: time of mild signs and symptoms

Invasive Phase: infection established and full signs and symptoms of the disease are found

called fulminating phase if signs/symptoms of invasive phase appear suddenly and severely

Acme Period (Climax): signs and symptoms are most intense

Decline Period: signs and symptoms subside → immune system gets the "upper hand"

Convalescence: body systems return to normal

Pathogen Exit: must be able to leave host to spread disease

need portal of exit

coughing/sneezing, removal of blood, open lesions, urine, feces, insect bites, etc.

Sequelae: after effects of a disease long after recovery

post polio syndrome

tertiary syphilis

shingles

Diseases caused by Viruses:

cause symptoms by directly killing cells

sometimes do not directly kill cells but cause infected cells to release many copies of the virus

viruses are lysogenic

symptoms of viral infections often caused by actions of immune system (release of WBCs, histamines, etc. that promote swelling, fever, etc.)

Diseases caused by Fungi:

infect in similar ways to bacteria

produce wide range of toxins

allergic reactions (inhalation of spores, etc.)

Infection and Disease on a Population Level:

Epidemiology: study of disease and patterns of the spread of disease

Etiology: the origin of a disease

Endemic: mild disease continually present in a given area/population → i.e. the flu

not a public health concern

Epidemic: disease causing a public health problem that is suddenly prevalent in higher than normal incidence in a specific geographic location

may be caused by change in virulence or introduction of infectious agent

and outbreak is a more contained epidemic

Pandemic: worldwide epidemic

Factors Affecting Disease Emergence and Reemergence:

changes in land use or agricultural practices

changes in human demographics

poor population health

pathogen evolution

contamination of food resources and water supplies

international travel

failure of the public health system

international trade

climate change