Nature of Infectious Diseases Biomedical Technology Understanding Infectious

Nature of Infectious Diseases Biomedical Technology

Understanding Infectious Diseases n Disease: the term refers to conditions that impair normal tissue function. Cystic fibrosis n Atherosclerosis n Measles n n All are diseases but there are fundamentally different causes for each disease

Infectious Diseases n 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) n Pathogens: microorganisms that are capable of causing disease. A true pathogen is one that causes disease in virtually every susceptible host.

Infectious Diseases n A true pathogen is an infectious agent that causes disease in virtually any susceptible host n Opportunistic pathogens are potentially infectious agents that rarely cause disease in individuals with healthy immune systems. n Diseases caused by opportunistic pathogens are typically found among people whose immune systems are failing n Examples: Cancer patients receiving chemo, people with AIDS or HIV positive

Infectious Diseases n The terms “infections” and “disease” are not synonymous. n An infection results when a pathogen invades and begin growing in the host n A disease results only if tissue function is impaired by the invasion and growth of a pathogen. polio virus n Ebola hemorrhagic fever n

Infectious Diseases n Virulent: degree or intensity of pathogenicity of an organism n Some pathogens are easily transmitted, or very contagious, but not likely to cause disease; they are not very virulent

Infectious Diseases The Polio virus is an example of not being very virulent: It probably infects most people who contact it, but only about 5 -10% of those infected actually develop clinical symptoms On the other hand, the Ebola hemorrhagic fever Is not transmitted easily by casual contact, but has a 50 -90% fatality rate if contacted. The worst infectious diseases are those that are both contagious and virulent!

Infectious Disease Process n In order for a pathogen to cause disease they must be able to: Enter the host body n Adhere to specific host cells n Invade and colonize host tissues n Inflict damage on host tissue n

Infectious Diseases n Entry to the host is usually through a natural orifice (mouth, eyes, genitals, wounds) n Attach to specific cells to invade other body tissues n Some pathogens inflict damage by growing; others produce destructive toxins; n An example is Corynebacterium diphtheriae that causes diphtheria. It only grows on nasal and throat surfaces; however, the toxin it produces gets in the blood stream and damages organs and nerve tissues.

Modes of Transmission n May be transmitted through direct or indirect contact n Direct contact: occurs when an individual is infected by contact with the reservoir n Indirect contact: occcurs when a pathogen can withstand the environment outside its host for a long period of time before infecting another individual n Horizontal transmission: agent is passed from person to person n Vertical transmission: agent is transmitted from mother to baby

Reservior: n Disease reservoir: the environment where the infectious agent survives Examples: humans or animals n Soil is the reservoir for Clostridium tetani that causes tetanus n Yersenia Pestis that causes the plague is rodents n

Reservior: n Infectious diseases that can be transmitted from animals to humans and humans to animals are known as “zoonoses” n These diseases account for more than 60% of emerging infectious diseases today n Recent examples include hantavirus, which are carried by rodents and can infect humans and SARS or Severe Acute Respiratory Syndrome; A contagious and sometimes fatal respiratory illness caused by a coronavirus

Microbes that cause infectious diseases n Bacteria: unicellular prokaryotic organism (no organized internal membranous structure) that reproduce by growing and dividing into two cells in a process known as “binary fission” a. bacillus (rod-shaped) b. coccus (spherical) c. spirillum (helical rods) Can be divided based on their gram stain reaction

Microbes n Gram negative bacteria appear pink after the staining procedure n Some gram negative organisms are: n Salmonella typhi that causes typhoid fever n Yersinia pestis which causes the plague

Microbes n Gram positive bacteria appear purple after the staining procedure n Examples are: n Staphylococcus aureus that causes skin, respiratory and wound infections

Microbes: n 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 n Viruses are classified using a variety of criteria including shape, size and type of genome n Examples: Herpes virus that causes chicken pox, cold sores and genital lesions and the poxvirus that causes smallpox

Microbes: n Fungi: eukaryotic with rigid cellulose walls and reproduce primarily by forming spores n Examples: Ringworm, Histoplasmosis (mild to severe lung infection transmitted by bat or bird droppings) n Protozoa: unicellular eukaryotes capable of a variety of rapid and flexible movements n Examples: Ringworm, Histoplasmosis

Microbes n Protozoa are unicellular, heterotrophic eukaryotes that are capable of rapid and flexible movements n Can be acquired through contaminated food or water or by the bite of an infected mosquito n Diarrheal disease can be caused by two common protozoan parasites, Giardia lamblia and Cryptosporidium parvum n Malaria is a tropical illness that causes 300500 million cases annually worldwide by Plasmodium

Microbes n Helminthes: simple, invertebrate animals that are infectious parasites. Drugs that kill helminthes are frequently toxic to humans. n Schistosoma, a flatworm causes swimmer’s itch in the U. S. , and schistosomiasis, which is a much more serious disease found in Africa and Latin America n Schistosome eggs hatch in water and the larvae infect snails. n The snails shed the larvae and attach and penetrate human skin

Microbes n The larvae then feed, grow and mate in the human tissues caused by the accumulating schistosome eggs; this results in disease symptoms of diarrhea and abdominal pain n Trichinosis is caused by a roundworm, Trichinella spiralis n Trichinosis comes from eating undercooked pork from infected pigs n Symptoms include vomiting, diarrhea and fever, and in severe cases heart failure and respiratory paralysis

Microbes n Prions consist only of protein n Diseases include Creutzfeldt-Jakob disease in humans, scrapie in sheep and “mad cow disease” in cattle n All prion diseases result in brain tissue that is riddled with holes n Some prion diseases are inherited, some are by eating infected tissue or through medical procedures such as tissue transplants