Microorganisms and Infectious Agents

Microorganisms and Infectious Agents

Microorganisms Definition

  • Microorganisms, also known as microbes, are tiny organisms usually invisible to the naked eye.
  • They can be unicellular, comprising just a single cell, multicellular, consisting of multiple cells, or acellular, not consisting of cells at all.
  • There can be millions of these microorganisms in a single gram of soil.

Classification of Microorganisms


  • Bacteria are prokaryotes, meaning they have no distinct nucleus.
  • They reproduce by binary fission, creating two identical daughter cells.
  • Bacteria can be beneficial, such as those in the gut aiding digestion, or harmful, causing diseases like tuberculosis.


  • Viruses are the smallest and simplest life form known.
  • They are acellular and can only reproduce by infecting host cells.
  • Viruses are responsible for various diseases, including HIV and influenza.


  • Fungi are eukaryotes that include organisms like yeast, moulds, and mushrooms.
  • They reproduce by spores and are essential for decomposing organic matter.
  • Some fungi can cause diseases, such as athlete’s foot and ringworm.


  • Protozoa are single-celled eukaryotic microorganisms.
  • Many protozoa are parasitic and depend on other organisms for survival.
  • Some protozoa can cause diseases, such as malaria (caused by Plasmodium species).

Infectious Agents

  • Infectious agents are microorganisms or prions that can spread within a host or between hosts.
  • These can be spread by various means, such as airborne droplets, food, water, insects, and direct contact.
  • Understanding infectious agents is crucial for controlling the spread of diseases.

Microorganisms Role in Disease

  • Many microorganisms maintain a symbiotic relationship with their host, neither benefiting nor harming them.
  • Some microbes are opportunistic pathogens and only cause disease when the host’s immune system is compromised.
  • Certain microorganisms are always pathogenic and can cause diseases such as tuberculosis, bubonic plague, and cholera.
  • Scientists use various microbiological techniques to identify pathogenic microorganisms, enabling appropriate treatment.

Interaction of Infectious Agents with the Immune System

  • The immune system is the body’s main defence against infectious agents.
  • It uses physical barriers like skin and mucous, chemical barriers such as gastric acid, and cellular responses to fight off infections.
  • White blood cells, or leukocytes, play a key role in the immune response by recognising and destroying pathogens.
  • Sometimes, the immune system can overreact, leading to allergic reactions, or it can fail to recognise self, causing autoimmune diseases.