Disease Causing Agents

Disease Causing Agents

Agent is the organism or force or substance whose presence or lack or excess causes disease. These are classified as:

• Mechanical agents : Friction, forces etc. which cause injury, sprain, dislocation and fracture.
• Physical agents : Light, heat, electricity, radiation, sound etc.
• Chemical agents : High or low levels of hormones, enzymes, physiological regulators.
• Nutrient agents : High or low levels of minerals, proteins, carbohydrates, lipids, vitamins.
• Biological agents (Pathogens) : Viruses, bacteria, fungi, protozoan, helminths, insects etc.

Germ theory of disease

Proposed by Louis Pasteur (1869) and “later established by Robert Koch (1876); states that” diseases do not occur spontaneously but are caused by organisms, the germs.

Pasteur discovered that anthrax disease in cattle is caused by Bacillus bacteria.

Koch’s Postulates

Robert Koch (1876) identified four basic criteria for an organism to be called as pathogen. (this does not apply to viruses, and leprosy bacteria

because virus and bacteria of leprosy cannot be cultured on artificial medium.)

They must be found regularly in the host’s body causing same symptoms.

Can be grown as pure culture in artificial medium out side the host’s body.

When inoculated in the same (or fresh) host it must cause same disease.

The same organism must be recovered from the inoculated host.

Pathogenicity (virulence)

The extent and intensity of the effect depends upon :

• Invasiveness : the ability of pathogen to penetrate host’s body.
• Toxigenicity : the capacity to generate toxic effect in host’s body.
• Sources or Reservoir : Soil, water and the living carrier (human or other animal).

Mode of transmission

• Direct Transmission : through contact, i.e. mother to child, through fomite (things of common uses e.g. soap, clothes, chair, handle of door, utensils etc.); air borne, like many viral diseases; droplet transmission. (sputum, phlegm).
• Indirect transmission – through secondary hosts (vector), carrier.