Communicable Diseases

• Pathogens are disease-causing viruses, bacteria, fungi or protists, which can infect animals and plants.
• Humans have an immune system, which can defend them from pathogens.
• Pathogens can be classified into four main types: Viruses, Bacteria, Protists, and Fungi.
• Viruses cause diseases such as HIV potentially leading to AIDS and Tobacco mosaic virus.
• Bacteria cause diseases such as Salmonella and Agrobacterium.
• Fungi cause diseases such as Athlete's foot and Rose black spot.
• Protists cause diseases such as Malaria and Downy mildew.
• All types of pathogen have a simple life cycle: they infect a host, reproduce themselves or replicate if it is a virus, spread from their host, and the organism they live on or in is a parasite.
• Pathogens also have structural adaptations that make them successful at completing their life cycles, which enable them to cause further disease.
• Diseases caused by pathogens are called communicable diseases.
• Communicable diseases are diseases that are caused by a pathogen and so are transmitted rather than inherited or caused by an environmental factor, meaning they can be transferred from one person to another.
• There are other types of disease which cannot be caught: inherited genetic disorders like cystic fibrosis.
• Deficiency diseases are diseases that develop because an organism (plant or animal) does not have enough vitamins or mineral ions, which are caused by a lack of essential vitamins or minerals.
• Minerals are naturally occurring, inorganic chemical substances necessary for both plant and animal health, such as scurvy.
• Scurvy is a deficiency disease that occurs when an individual has insufficient vitamin C.
• Diseases like cancer develop as a result of exposure to carcinogens.
• Carcinogens are chemical or other agents that cause cancer or develop naturally as cell division occurs incorrectly.
• All organisms are affected by pathogens, even bacteria are infected by certain types of virus.
• Some infections can be transferred to organisms of a different species.
• Transmission of pathogenic disease can occur in a number of important ways, as shown in the table below.
• Direct contact can be sexual contact during intercourse or non-sexual contact, like shaking hands.
• Water can transmit many diseases, such as the cholera bacterium.
• Airborne diseases can be spread when a person who is infected by the common cold sneezes, spraying thousands of tiny droplets containing virus particles to infect others.
• Unhygienic food preparation can cause bacterial diseases like Escherichia coli which is a cause of food poisoning.
• Any organism that can spread a disease is called a vector.
• Many farmers think tuberculosis in their cattle can be spread by badgers.