We have seen useful microorganisms so far; however, some microorganisms are harmful. They can make the host cell, for example, a plant, an animal or a human being, unwell.
Some microorganisms can cause illness in the host cell. Such microorganisms that cause illness to living organisms or host-cell are called pathogens.            
Some important disease-causing pathogens are:
  • Mycobacterium tuberculosis, causes tuberculosis (TB)
  • Streptococcus mutans is mostly found in the teeth (oral cavity) of human beings and causes tooth decay.
Mycobacterium tuberculosis
Streptococcus mutans
Communicable diseases
Diseases that can spread from an infected person to a healthy person are called communicable diseases.
Some communicable diseases are the common cold, cholera, typhoid, dengue, etc.
Let's say if a person is suffering from a common cold with sneezing and coughing. After sneezing, the droplets of the infected person have thousands of virus particles, which can be transmitted into another person's body if they are close to each other.

Diseases like dengue are transmitted from one person to another person through carriers such as mosquitoes. The transmission is not by the droplets of the infected person but through the blood. The most important carriers of dengue are female anopheles mosquitoes. The transmission of the disease can be either through blood or through food particles, over which the infected mosquito sits. 

Female anopheles mosquito
Some diseases caused by microorganisms in human beings
Diseases caused by microorganisms in animals
Microorganisms can also cause disease to animals.
  • One of the deadliest diseases in animals is anthrax, caused by Bacillus anthracis.
  • Some viruses cause foot and mouth diseases in animals.
  • A virus causes rabies, and Q-fever is caused in animals by bacteria.
Vaccines will be helpful to prevent animals from getting affected by rabies.
Diseases caused by microorganisms in plants
  • In plants, few fungi cause diseases like citrus canker and clubroot.
  • Spotted wilt and mosaic are some of the viral diseases caused by microorganisms.
  • Aster yellow and scab are diseases caused by bacteria in plants.
Canker is mainly found in citrus fruits such as lemon, oranges known as citrus canker.