Disease and Pathogens Explained

Disease is a problem with a structure or process in the body. Diseases are sometimes caused by pathogens, these types of diseases can often be passed from person to person and are known as communicable diseases. 

Health is defined by the World Health Organisation as:

“a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity”

Health is related to disease in that a healthy person would not have any disease.

A pathogen is an organism that causes disease after entering the body and bypassing its physical and chemical defences. At this point, the immune system starts to act and attacks the invading pathogen.

Four types of pathogens cause communicable diseases:

• Bacteria
• Fungi
• Viruses
• Protists

Not sure what protists are? Learn more about the classification of organisms.

At the GCSE level, you are expected to understand the methods of transmission (how a disease is spread) and what steps can be taken to reduce transmission.

The main transmission methods are airborne, waterborne, animal vectors and then physical/sexual contact.

• Airborne transmission involves the spread of either spores in the case of fungi or cells for viruses and bacteria in the air. These are then ingested or come into contact with other organisms and cause the disease.
• Waterborne transmission involves people consuming contaminated water, the most famous example is cholera which causes vomiting and diarrhoea, which then contaminates water supplies and spreads the disease.
• Animal vector transmission involves an animal host passing on the disease to a different species. The Bubonic Plague (Black Death) is caused by a bacterium that lives inside fleas, which in turn feed on rats’ blood. When they bite humans, the bacteria and the disease are passed on.
• Physical contact transmission (called mechanical contact for plant diseases) is where actual contact between people passes on a pathogen, this can be sexual contact or blood contact. AIDS is caused by a virus which can be transmitted through sexual contact, blood transfusions or through sharing of needles.

A segregation camp in Karachi, Pakistan (1897). After an outbreak of bubonic plague (The Black Death), individuals with the disease were segregated from society to prevent the spread of the disease. This is a common approach with highly contagious diseases like Ebola (Hemorrhagic Fever) which spread through air, water or physical contact. 

Other methods of reducing the transmission of disease are:

• mosquito nets and repellants for malaria.
• destroying and disposing of infected plants (see table below for plant diseases)
• fully cooking food – bacterial diseases like food poisoning and stomach ulcers
• coughing and sneezing into tissues and covering your mouth – tuberculosis and other airborne diseases.
• cleaning and purifying drinking water – cholera

The table below outlines some important common diseases, how they are transmitted and what exam board they are relevant to. 

Test your knowledge of diseases, pathogens and how the body prevents them with the quizzes below!