6.3.2 Communicable Diseases

• 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
• 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

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
• 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
• 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

• The prevalence of communicable disease can be improved in two main ways
  • Prevention
  • Reduction
• 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
    • A well-educated population will recognise the risks of disease spreading and take the necessary precautions
• 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 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
  • 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