Disease transmission

Communicable diseases can pass between hosts. In animals and plants, these diseases may be caused by pathogens: viruses, bacteria, protists or fungi. A host is an organism that a pathogen can infect.

Transmission occurs when a pathogen passes from an infected host or a contaminated source to another host. If the pathogen enters and establishes itself in the new host, it can cause an infection. The route by which it travels depends on the pathogen and the host.

Routes of disease transmission

A pathogen can reach a new host by several routes:

• Direct contact: the pathogen transfers when hosts touch.
• Water: water carrying the pathogen reaches a new host.
• Air: droplets or particles carrying the pathogen reach animal or plant tissues.

Breaking a transmission route

A pathogen has fewer opportunities to cause infection if its route to a susceptible host is interrupted. For example, isolating an infected host reduces its contact with other hosts. Removing infected plants can likewise limit contact between a pathogen and plants that could be infected.

Rose black spot

Rose black spot is a fungal disease that infects roses. Purple or black spots develop on infected leaves. The leaves often turn yellow and fall off early.

Damaged leaves and the loss of leaves reduce photosynthesis, the process by which the plant makes glucose using light. With less glucose available for growth, the rose grows less.

The fungus can spread through the environment in water or wind. Rose black spot can be treated with fungicides and by removing and destroying affected leaves.