Communicable diseases, disease prevention, the immune system

• What disease: Condition which impairs normal functioning of organism; disorder can be due to a single cause or be multifactorial (Single: COVID or Multifactor: Depression), it is characterised by symptoms that are physical, mental or both
• Reproduction Rate = R_0 = (k)x(d)x(p)

  • R_0 - AVG. Num of people each infected person will infect
  • k - number of contacts a person has during 1 day
  • d - duration of infectiousness for a person
  • p - probability of transmission/contact
• Infectious diseases go, as pathogens, from one host to another; mostly microbes (e.g. bacteria, viruses, protozoa and fungi), some multicellular parasites (e.g. worms); not all microbes infect, majority are harmless and many beneficial
• Organisms infect a host and multiply, but don’t always cause disease: parasites; organism living in (endoparasites) or on (ectoparasites) the host and does damage; a ‘successful’ parasite lives unnoticed by its host and does little damage
• Parasite burden may become too great, cause damage, enable secondary infections by other microbes to occur and this results in disease
• Pathogens get into and infect plants through the stomata, cuts/damage to bark/lignin, roots, damaged leaves or an insect vector
• Disease symptoms are from microbe’s ability to damage host cells and tissues (pathogenicity); ability to cause disease depends on susceptibility/resistance of the host (dictated by genes, if they’re very young/old or immunosuppressed)
• Some symptoms of disease (e.g. fever and inflammation in humans) are due to the immune response the pathogen induced
• A Successful pathogen can

  • ADHERE to host cells; e.g. using pili and gummy secretions such as slime layers and capsules (bacteria) or surface receptors (viruses)
  • INVADE its host tissues; using chemicals such as enzymes that damage tissues and cells enable penetration
  • COLONISE specific tissues, overcome host immune system and multiply
  • Produce VIRULENCE FACTORS or TOXINS; which protect against host defenses or increase access to host nutrients
• Many animals, including humans, have evolved a wide range of defences against disease, such as; barriers, antimicrobial secretions, phagocyte white blood cells, and antibodies
• Plants do not have antibodies and white blood cells but they do recognise microorganisms plants make antimicrobial compounds and have programmed cell death (apoptosis) – kill off their own infected cells; therefore if the fungus or virus that is causing disease needs the plant alive to get nutrients, then the dead cells form a barrier and stop infection
• Transmission

  • Direct transmission; physical contact (e.g. body fluids, childbirth, fomites) or droplet infection (e.g. coughing, sneezing, inhalation)
  • Indirect transmission; Vector (e.g. mosquito, sandfly, tsetse fly) or oral transmission (e.g. eating or drinking contaminated food or water)
• Specific Diseases


  Tuberculosis - Droplet infection

  Bacterial Meningitis - Droplet Infection

  HIV/AIDS - Direct Physical Contact

  Influenza - Droplet Infection

  Malaria - Vector, and Direct Physical Contact

  Ring Worm - Direct Physical Contact (Skin to skin)

  Athlete’s Foot - Direct Physical Contact
• Pathogens get in by portals of entry, most significant human ones;

  • Respiratory system e.g. nose and mouth
  • Genito-urinary system
  • Digestive system
  • Breaks in the skin surface

  All have physical, chemical and immunological defences, but may be breached

  A) skin

  B) Insect bite

  C) Anus

  D) Ear

  E) Conjunctiva

  F) Nose

  G) Mouth

  H) Placenta

  I) Vagina

  J) Penis

  K) Urethra