Communicable Diseases

• Pathogens:

  • Communicable diseases are spread by pathogens
  • A pathogen is any microorganism that causes disease in another organism (e.g. in plants or animals)
  • 'patho-' = disease
  • '-gen' = generating
  • Many microorganisms are pathogens, including:
  • Bacteria
  • Fungi
  • Protists (protoctists)
  • Viruses
• Pathogens:

  • Very few species within these groups (apart from the viruses) are pathogens, as many bacteria, fungi and protists are harmless and do not cause disease
  • However, all viruses are pathogenic as they can only exist by living inside the living cells of other organisms (or by using these cells to create more viruses)
• Pathogenic bacteria:

  • Pathogenic bacteria do not always enter the hosts' cells, they can remain within body cavities or spaces
  • Toxins produced by the bacteria also damage cells
  • They are small and can reproduce very quickly
  • Bacterial infections include:
  • Mycobacterium tuberculosis causes tuberculosis (TB) in humans
  • Neisseria meningitidis causes bacterial meningitis in humans
  • Helicobacter pylori causes stomach ulcers
  • Vibrio cholerae causes cholera in humans
• Pathogenic fungi:

  • Fungal diseases are much more common in plants than animals
  • Fungi can be single-celled or multicellular (with threads of hyphae)
  • The spores they produce allow them to infect other organisms
  • In plants, fungal diseases tend to be much more serious and can threaten entire crops
  • Fungal diseases include:
  • Cattle ringworm and athletes foot in animals
  • Black Sigatoka in bananas
  • Chalara Ash Dieback in ash trees
• Pathogenic protists:

  • Protists are a diverse group of eukaryotic (and usually single-celled) organisms
  • They are parasites which means they need a host in order to survive
  • Only a small number of protists are pathogenic, but the diseases they cause are often serious
  • Examples of diseases caused by protists are:
  • Plasmodium falciparum is a protist that causes severe forms of malaria in humans
  • P. infestans causes the infamous potato blight
• Modes of Transmission - Communicable Diseases:

  Transmission of Pathogens-

  • Pathogens can spread from one individual to another in the following ways
  • Via water
  • Either drinking or bathing in dirty water 
  • Example: cholera
  • The pathogen travels via faeces
  • Via the air
  • Spores can be distributed in the wind
  • Example: Barley powdery mildew
  • Erysiphe graminis is the pathogen
• Modes of Transmission - Communicable Diseases:

  Transmission of Pathogens-

  • Via body fluids
  • Blood e.g. sharing needles
  • Semen during sexual contact
  • Some diseases spread via breast milk from mother to infant
  • Example: HIV
  • Through contact
  • Example: Athlete's foot is a fungus
  • Spreads when a person steps on the same surface as an infected person
  • Example: Covid-19 can be transmitted via contaminated surfaces
• Modes of Transmission - Communicable Diseases:

  Transmission of Pathogens-

  • Through ingestion
  • Example: Salmonella and other types of food poisoning can be transmitted via eating poorly cooked or too-old food
  • Via animal vectors
  • Example: P. falciparum (the malarial protist pathogen) is transmitted by an animal - the mosquito
  • Via the soil
  • Example: Crown gall disease caused by the bacterium Agrobacterium tumefaciens
• Reducing the Spread of Disease:

  • The prevalence of communicable disease can be improved in two main ways
  • Prevention
  • Reduction
• Reducing the Spread of Disease:

  • Behaviour in social situations can affect the spread of disease
  • The density of population and housing conditions
  • A densely-packed neighbourhood in a city, close to sewage flow and waste food/garbage can increase risk of diseases spreading
  • Poor diet can bring about deficiency diseases or can cause greater susceptibility to infections
  • Availability of good healthcare in a neighbourhood affects the spread of disease
  • The level of education in a community is important
• Reducing the Spread of Disease:

  • A well-educated population will recognise the risks of disease spreading and take the necessary precautions
• Reducing the Spread of Disease:

  • Individual behaviour
  • Personal hygiene such as handwashing, wearing a mask if suffering from a cold can reduce the spread to others
  • Self-isolating when sick
  • Thorough cleaning of the home and surfaces shared by others e.g. kitchen work surfaces
  • Consulting healthcare professionals when sick to get early detection and treatment will prevent spread
  • Being vaccinated e.g. against 'flu or Covid-19 will reduce the spread to others
  • Medicine, agriculture and the public all have their parts to play in disease detection
• Detection of the antigen:

  • Prevention is better than cure
  • If antigens can be detected in a person's bloodstream or body fluids, this may lead to early diagnosis and prophylaxis (measures taken to prevent onset/spread of the disease, rather than curing the full-blown disease)
  • An example is the Covid-19 lateral flow kits that were distributed widely in the 2020-2022 pandemic
  • They used monoclonal antibodies to detect the presence of a Covid-19 antigen in mucus lining the airways
  • Prophylaxis was immediate compulsory self-isolation to minimise spread to others
• DNA testing:

  • Genome testing can detect the presence of alleles that could indicate disease at a later time in a person's life
  • Such knowledge allows individuals to amend their lifestyle / diet etc. in order to be able to prepare for life with a non-communicable genetic disorder
  • An example is in families with a history of colo-rectal cancer, in order for younger members of the family to be aware of the likelihood of them developing the condition when older
• DNA testing:

  • DNA testing can inform doctors about a person's likely response to drug treatments for other diseases and help to prescribe an effective dose
  • New-born baby screening
  • In the USA, all new-born babies are routinely screened for the following serious conditions:
  • Congenital hypothyroidism
  • Sickle cell disease
  • Phenylketonuria (PKU)
  • This is done so that therapy can begin as soon as possible
• Visual identification of the disease:

  • Trained medical professionals can spot the early signs of disease, whereas ordinary members of the public may not be able to
  • Public education programmes exist in order to make the public aware of how to spot certain diseases in order to act quickly
  • Meningitis in babies and young children is a good example; often by the time a doctor or nurse can see a patient the condition has deteriorated so far that the child dies
• Visual identification of the disease:

  • A well-informed parent/carer can spot the signs and rush the child to the hospital, where the disease can be treated successfully in its early stages before it develops too far
• Reduction in plant disease spread:

  • Park rangers, walkers, nature enthusiasts, farmers, woodsmen, gardeners, horticulturalists etc. can all report incidences of disease in plants to alert the authorities 
  • For example, the UK Forestry Commission has an online tool called TreeAlert that allows anyone to report a new case of Ash Dieback, a serious disease afflicting native ash trees
  • Users are encouraged to report online with photos and measurements, so appropriate remedial action can be taken swiftly