Communicable Disease

Pathogens are disease-causing microorganisms. 
There are four types of pathogen:

• Bacteria
• Virus
• Fungi
• Protoctista.

All of them work in very different ways, but they all cause communicable disease. 

Communicable disease definition:
"An infectious disease transmissible by direct and indirect contact.

• Only some bacteria are pathogens.
• They are prokaryotic cells
• They have no nucleus or membrane bound organelles.

They can be classified in two ways:

SHAPE
There are many different shapes of bacteria, this includes rod, corkscrew, and spiral.

CELL WALL
The gram staining method can be used to distinguish between the two types of cell wall.
Gram positive - appears purple/blue
Gram negative - appears red.

• Very small - between 0.02 and 0.3 micrometers
• They are no living.
• Reproduce by taking over a living cell.
• They reproduce rapidly.
• The structure of a virus is DNA surrounded by a protein coat.

• Mainly cause disease in plants.
• Multicellular eukaryotes.
• Some fungi feed on dead/decaying organisms - these are called saprophytes.
• They spread rapidly.

• They are single-celled eukaryotes.
• They either require a vector to be transferred or water.

1. They take over host cells metabolism.
2. Virus DNA enters the host cell.
3. The host cell is used to make new viruses.

• It takes over the cell but not the genetic material
• It digests the cell and uses its contents.

• They digest and destroy living cells.
• They produce toxins.
• They cause symptoms of disease.

• Produce toxins which cause symptoms.
• They poisons or damage the host cells.

• Direct contact - Skin to Skin
• Inoculation - Pierce the skin
• Ingestion - By food and water.

• Poor nutrition
• Overcrowding
• Climate change
• Poor immune system
• Socioeconomic factors

• Lack of minerals in the soil.
• Overcrowding increases likelihood of direct contact
• Climate change - results in new vectors.
• Planting varieties of crops susceptible to disease.
• Good conditions for pathogens - damp and warm. 

• Waxy cuticle
• Bark
• Callus
• Cellulose
• Lignin

Unlike animals, plants don't have an immune system. Instead, they have cell signalling which initiates defence measures and causes communication.

Cellulose and callus may appear to have the same structure, however, they differ slightly by the positioning of their glycosidic bonds. Callus has 1,4 whereas cellulose has 1,3.

• Callose synthesised
• Deposited next to infected cells in between the cell wall and cell membrane.
• Callose is still added in large quantities after initial infection.
• Lignin added for strength.
• Callose blocks off sieve plate, cutting off infected part.
• Callose deposited in the plasmodesmata, the area between infected cells and their neighbours to prevent further spread of the pathogen.

• Plant responses are quick as they are not passive.
• Plant cell receptors detect molecules/chemicals released by pathogens.
• Pathogenic enzymes break down cell wall and recognise products.
• Callose and lignin produced to strengthen cell wall.
• Defensive chemicals alert neighbouring cells.
• Some defensive cells can attack pathogens directly.

• Insecticides: Caffeine
• Antibacterial compounds: Phenols
• Antifungal compounds: Gossypols
• Insect repellent: Pine resin
• Antioomycetes: Glucanases 

SKIN
Sebum lowers pH of skin - inhibits pathogen growth.
Sweat creates a salty environment which is hard for bacteria growth.

TEARS
Contain lysozymes which destroy bacteria.

EYELASHES
Keep out substances by blinking.

MUCUS
Traps microorganisms
Also, contains lysozymes which destroy bacterial and fungal cell walls.

DIGESTIVE SYSTEM
Lowers pH of the stomach due to the prescence of stomach acid (hydrochloric acid) providing a difficult environment for microorganisms.

EXPULSIVE REFELEXES
Diarrhoea, Vomitting, Sneezing and Coughing.

• Cut forms
• Pathogens can enter the body.
• Clots seal wound.
• Platelets and collagen come into contact.
• Platelets secret thromboplastin which starts off many reactions and serotonin which causes smooth muscle to contract, making blood vessels constrict resulting in the narrowing of blood vessels.
• The clot dries and scabs form.
• Epidermal cells start to grow below the scab, permanently sealing the wound.
• Collagen strengthens the new tissue.
• Scab falls off when epidermis reaches normal thickness.

• Mast cells activated
• Histamines and cytokines released
• Histamines cause dilation of blood vessels, causing redness and heat which inhibits pathogen reproduction.
• Histamines make blood vessel walls leaky, which forces blood plasma out (now called tissue fluid) causing swelling.
• Cytokines attract phagocytes to the area.
• These dispose of the phagocytes by phagocytosis.