1. Emerging & Re-emerging Infectious Disease

2. Infectious Diseases
Disease: conditions that impair normal tissue function
Genetic or Metabolic diseases: ex Cystic Fibrosis
Disease of aging: ex atherosclerosis
Infectious Disease: caused by the invasion of a host by agent’s whose activities harm the host’s tissues

3. Pathogens: microorganisms that are capable of causing disease
Opportunistic pathogens are potentially infectious agents that rarely cause disease in individuals with a healthy immune system.

4. The terms infection & disease are not synonymous
An infection results when a pathogen invades and begins growing in a host
Disease results only if and when, as a consequence of the invasion and growth of a pathogen, tissue function is impaired

5. Contagious vs. Virulent
Some infectious agents are easily transmitted (very contagious) but are not likely to cause disease (not very virulent)
Example: Polio-probably infects most people it contacts, however only 5-10% actually develop the disease.

6. Other infectious agents are very virulent, but not terribly contagious.
Ex: Ebola hemorrhagic fever, virulence of the virus is 50-90% fatality among those infected, however the virus is not transmitted easily by casual contact.

7. How do infections happen?
In order to cause disease, pathogens must be able to enter the host body, adhere to specific host cells, invade and colonize host cells, invade and colonize host tissues, and inflict damage on those tissues
Pathogens usually enter through natural orifices or through breaks in the natural skin barrier such as wounds.

8. Corynebacterium diphtheria
Bacteria that causes diphtheria
Grows only on nasal and throat surfaces
Damage to tissues is due to the production of enzymes or toxins
Toxin is distributed to other systems via the circulatory system resulting in damage to the heart, liver and nervous tissues

9. Streptococcus pyogenes
Infectious agents associated with “strep throat” and “flesh eating disease”
Produces enzymes that break down barriers between epithelial cells and remove fibrin clots, helping the bacteria invade tissues

10. Microbes 5 major types of infectious agents Bacteria Viruses Fungi
Protozoa
Helminths
In addition, a new class of infectious agents, the prions, has recently been recognized

11. Bacteria
Have no organized internal membranous structure such as nuclei, mitochondria, or lysomes.
Most bacteria reproduce by growing and dividing into two cells (binary fusion)

12. Variety of morphologies. 3 most common
Bacillus (rod shaped)
Coccus (round)
Spirillum (helical rods)

13. Some bacteria require oxygen Some bacteria can not tolerate oxygen
aerobes
Some bacteria can not tolerate oxygen
anaerobes
Some bacteria can grow either with or without oxygen
facultative anaerobes

14. Gram Stain
Bacteria are generally divided into 2 broad classes based on their cell wall structure, which influences their Gram stain reaction
Gram Negative bacteria appear pink after the staining procedure

15. Gram Negative Bacteria
Salmonella typhi (causes Typhoid Fever)
Yersinia pestis (causes the plague)

16. Gram positive bacteria
Appear purple after gram stain procedure
Staphylococcus aureus (causes skin, respiratory & wound infections)
Clostridium tetani (produces a toxin that can be lethal for humans)

17. Viruses
Not organisms themselves, cannot reproduce apart from the host cell
Cause disease by disrupting normal cell function
Classified using a variety of criteria including shape, size, and type of genome.

18. Viruses Examples of viruses:
Herpes viruses that cause chicken pox, cold sores and genital herpes
Poxvirus that causes smallpox
Rhinovirus that causes the common cold
Myxoviruses and paramyxoviruses that cause influenza, measles and mumps
Rotaviruses that cause gastroenteritis
Retroviruses that cause AIDS and several types of cancer

19. Fungi Reproduce primarily by forming spores
Together with bacteria, fungi work to be the “decomposers” in our environment
Examples of diseases caused by fungi
Ringworm
Histoplasmosis (lung infection caused by bat or bird droppings)

20. Fungi
Yeasts of the Candida genus are opportunistic yeast infections and cause diseases such as vaginal yeast infections and thrush (a throat infection) among people who are immunocompromised or undergoing antibiotic therapy
Antibiotics reduce the bacterial population normally present in the throat and vagina, allowing the yeast to grow unchecked

21. Protozoa
Do have cell walls so are able to have rapid and flexible movements
Can be acquired through contaminated food or water or by a bite of an infected anthropod such as a mosquito

22. Protozoa Diseases
Giardia lamblia & cryptosporidium parvum are two protozoa parasites that cause diarrheal disease in the US
Malaria, a tropical illness, is caused by several species of the protozoan Plasmodium

23. Helminths
Are simple, invertebrate animals, some which are infectious parasites
Because they are animals, their physiology is similar to humans, making this parasite difficult to treat because drugs that kill helminths are frequently very toxic to humans

24. Helminths Have complex reproductive cycles and many require a host
Examples:
Schistosoma, Caused by a flat worm
Results in diarrhea and abdominal pain
Trichinella spiralis, caused by a roundworm
Results in vomiting, diarrhea and fever, later symptoms include muscle pain, congestive heart failure and respiratory paralysis

25. Prions Infections particles that consist only of proteins
Pronounced (pree-ons)
Linked to some CNS degenerative disorders in humans and animals

26. Prions Examples of diseases: Creutzfeldt-Jakob disease in humans
Scrapie in sheep
Bovine spongiform encephalopathy (“mad cow disease”) in cattle
Some prion diseases are inherited, others are caused from eating infected tissue

27. Epidemiology
Defined as the study of the occurrence of diseases in populations
Epidemiologist are concerned not only with infectious disease but also with non-infectious disease such as cancer and environmental diseases such as lead poisoning

28. Disease reservoirs
The reservoir for a disease is the site where the infectious agent survives
Example: Humans are the reservoir for the measles virus because it does not infect other organisms
Wild rodents are the reservoir for Yersinia pestis, the bacteria that causes the plague
Soil is the reservoir for the bacteria Clostridium tetani which causes tetanus

29. Modes of transmission Transmitted by either direct or indirect contact
Direct contact is when an individual is infected by contact with the reservoir
Example: touching an infected person, eating infected meat, being bitten by an infected animal, inhaling infectious droplets from a sneeze or cough, intimate sexual contact

30. Direct contact diseases
Some diseases spread by direct contact include:
Ringworm
AIDS
Trichinosis
Influenza
Rabies
Malaria

31. Modes of Transmission
Indirect contact occurs when a pathogen can withstand the environment outside its host for a long period of time before infecting another individual
Examples: tissues or toys handles by sick people, ingesting food or beverages contaminated by contact with a disease reservoir

32. Indirect contact
Fecal-oral route is a significant form of indirect transmission for gastrointestinal diseases such cholera, rotavirus infection, cryptosporidiosis, and giardiasis.
Also included in indirect transmission are diseases transmitted from parent to child such as AIDS and herpes encephalitis

33. Role of research in prevention
Understanding the infectious cycle is critical in order to identify accessible targets for control strategies
Examples: Vector-borne diseases may be prevented by control methods that either kill the vector or prevent it’s contact with humans. Vaccines may prevent the development of a pathogen within a host, drugs may be used to prevent infections or suppress the disease process

34. Host Defenses against Infectious Disease
The humans body has several mechanisms for preventing diseases
These include anatomical barriers to invading pathogens (skin, bone, nasal passage), physiological deterrents to pathogens (tears, sweat, blood, vaginal secretions), and the presence of normal flora (microorganisms present on and in the body)

35. Immunity
When a host encounters an antigen that triggers a specific immune response for the second or later time, the memory lymphocytes recognize it and quickly begin growing and dividing. The immune response occurs so quickly the second time that the pathogen does not have time to reproduce before the body has destroyed it.

36. Vaccination
A vaccine is either a killed or weakened strain of a particular pathogen
The body’s immune system will respond to these vaccines as if they contain the actual pathogen.
As a result, memory lymphocytes will rapidly respond when the actual pathogen is encountered, destroying the pathogen before it can reproduce

37. Vaccination Many diseases are not easily prevented by vaccination
Some pathogens have a way to evade the immune response either by not allowing antibodies to pass through their cells, by disguising themselves as a host cell or by mutating.
Cold and influenza viruses are examples of rapidly mutating pathogens

38. Public Health measures to prevent Infectious Disease
Safe water: Many pathogens that cause gastrointestinal diseases (cholera & typhoid fever) are transmitted in water.
Travelers to developing countries are asked to be immunized
Unnecessary in the US and developed countries because the water used for drinking, washing and preparing food is purified before it goes into homes.

39. Water purification
Purification methods include settling, filtration and chlorination.
Well water is usually safe if guidelines about distance from sewage disposal facilities is followed.
When purification systems breakdown, such as with unusual flooding, drinking water may not be safe and should be boiled or treated with chlorine prior to ingesting.
Public water must be protected from sewage

40. Sewage treatment and disposal
Sewage includes wash water, water from toilets and storm run-off
These fluids may carry pathogens for many waterborne diseases, including giardiasis and hepatitis A.
To ensure safety, the US government requires that sewage be treated to eliminate pathogens

41. Food Safety programs
The US has many standards, inspection plans and regulations about food preparation, handling and distribution.
Meat packing plants, restaurants and supermarkets are all inspected
Milk is pasteurized and dated for sale and is analyzed periodically for contamination
Canning and preserving foods is maintained through quality control checks.

42. Animal control programs
Aniamls are carriers of many diseases that also affect humans
Inspecting herds animals for tuberculosis (due to the bacterium Mycobacterium bovis) and brucellosis (a disease that causes spontaneous abortion in herd animals and abscesses of the liver, spleen, bone marrow and lymph nodes in humans) has helped prevent passage of those organisms in milk and infected meat

43. Animal Control programs
Before dogs can be licensed, owners must show proof of rabies vaccination
Most cases of rabies in the US is due to bites from wild and stray animals, these animals are usually impounded and destroyed
Bubonic plague is spread by rats. Rat control in urban areas is a major component of public health efforts

44. Animal control measures
Insect-borne diseases such as malaria can be controlled by eliminating breeding areas for insects and using pesticides.
Examples: draining areas where stgnant water collects

45. Vaccination Programs
Most states now require parents show proof of vaccination before enrolling their children in daycare or public schools.
If a certain proportion of the population is immune to the disease, the pathogen that causes the disease will be unable to reproduce itself at a high enough level to maintain itself in the population. Eventually the pathogen cannot spread any further and may be eliminated from the population.

46. Public Health Organizations
City and state agencies are responsible to enforce regulations, provide public health services such as vaccinations, and monitor and report the incidence of a particular disease to state and federal agencies.
All these agencies report data to the US Public Health Service

47. Public Health Organizations
NIH: National Institute of Health. One of the 8 health agencies of the US Public Health Service
Supports health related research aimed at understanding, preventing, treating and controlling infectious and other diseases of humankind

48. Public Health Organizations
Centers of Disease Control (CDC)
In Atlanta, Ga. Investigates disease outbreaks, publish epidemiological reports, sponsor a variety of educational programs, research projects and reference laboratories
Food & Drug Administration (FDA)
Monitors the safety of our food, medicines, and many other products we use daily
World Health Organization (WHO)
Provides international surveillance and control of disease

49. Treatment of infectious diseases
Antibiotic literally means “destoyer of life”
Antibiotics refer to a chemical substance used to treat bacterial infections
Antimicrobial refers to anything that inhibits the growth of microbes which does not include antihelminthics because worms are not microscopically small

50. Antibiotics Drugs used to treat bacterial diseases
Grouped into categories based on their modes of action
In general, these drugs inhibit cell wall synthesis, nucleic acid synthesis, or other enzyme-catalyzed reactions
Examples: penicillins, cephalosporins, tetracycline and erythromycin
Some antibiotics can be toxic to human tissues when used in high doses or prolonged periods of time

51. Antibiotics Rifampicin-used to treat TB (tuberculosis)
Sulfonamides interfere with the synthesis of folic acid, a vitamin necessary for nucleic acid synthesis. Most bacteria must synthesize their own folic acid because their membranes are impermeable to external folic acid.

52. Treatment of Viral Diseases
In general, drugs that effectively inhibit viral infections are highly toxic to host cells because viruses use the host’s metabolic enzymes in their reproduction.
For this reason, most viruses are treated symptomatically until the host’s immune system controls and eliminates the virus or the host dies.

53. Antiviral drugs Drugs that are typically target virus specific
Acyclovir-used to treat outbreaks of genital herpes
Amantadine-used to prevent or moderate influenza among those at high risk of severe illness from the disease

54. Antiviral Drugs AZT-inhibits that replication of the HIV genome
AIDS patients today are prescribed proteases that interfere with the packaging of the HIV genome into virus particles

55. Treatment of fungal and parasitic disease
The development of drugs to treat fungal, protozoan, and helminthic diseases is challenging because the agents that kill or inhibit growth in the organisms are also highly toxic to human cells.
Amphotericin B is an antifungal that disrupts the cell membrane
Antihelminthic drugs target the adult worm

56. Protozoa Treatment Malaria was successfully treated with chloroquine
In recent decades, Plasmodium species that are resistant to this drug have appeared and spread to areas where malaria is a common threat. In those areas, a combination of the drugs sulfonamide and pyrimethamine is frequently used to treat the disease

57. Resistance to antimicrobial agents