Infection and Disease

Relationships Between Hosts and Microorganisms:Mutualistic: both partners benefiti.e. gut microflora → bacteria receive warm nutrient rich habitat, synthesize vitamin K for the hostCommensal: neither host nor microbe are harmedi.e microbial skin inhabitantsParasitic: one partner benefits, the other is harmedmicrobe based diseaseDisease-Causing Microbes:Pathogen: a disease-causing microbe → parasitic microbePathogenicity: microbe's ability to enter a host and cause diseaseVirulence: 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 spreadsuccessful invasiveness requires pathogens to have virulence factorsVirulence 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 virulenceTransposal: passage of a human pathogen through a different animal attenuates (weakens) the pathogen so it can be used as a vaccineParasitism: pathogens cause damage and disease in hostToxigenicity: 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 proceduresIdiopathic Diseases*: unknown causesNosocomial 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 tissueEndogenous Infection: normal microbiota enter sterile tissueOpportunistic Infections: commensals take advantage of a change in the body's environment that favours the microbei.e. takes advantage of a change in the body/immune systemPrimary Infections: occur in otherwise healthy bodiesSecondary Infections: occur in bodies weakened by primary infectionsLocal Diseases: restricted to a single area → i.e. boilsSystemic Diseases: disseminate to organs and systemsCommunicable Diseases: contagiousNoncommunicable Diseases: not easily transmitted to another hostacquired directly from environment → i.e. tetanusSepticemia: bacteria proliferate in the bloodBacteremia/Viremia: pathogens transported by blood but do not multiply in itThe 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 benefitCommensalism: 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 othersSymptoms: characteristics of disease only observed/felt by the patientSyndrome: combination of signs and symptoms that occur together → indicative of a particular diseaseIncubation Period: time between entry of the microbe and symptom appearance → little or no ill effect notedProdromal Phase: time of mild signs and symptomsInvasive Phase: infection established and full signs and symptoms of the disease are foundcalled fulminating phase if signs/symptoms of invasive phase appear suddenly and severelyAcme Period (Climax): signs and symptoms are most intenseDecline Period: signs and symptoms subside → immune system gets the "upper hand"Convalescence: body systems return to normalPathogen 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 diseaseEtiology: the origin of a diseaseEndemic: mild disease continually present in a given area/population → i.e. the flunot a public health concernEpidemic: 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 epidemicFactors 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