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Pathology defines disease as a specific pathological condition that occurs when the normal harmony of living cells within the body gets infected by germs.

Examples of disease-causing agents are prions (small proteinaceous infectious agents), helminths (large macroparasites or parasitic worms), bacteria (single-celled or prokaryotic microorganisms), viruses (microscopic infectious pathogens), and fungi (parasitic microorganism).

The different kinds of diseases that pathogens cause are measles, common cold, flu, typhoid, chickenpox, genital herpes, AIDS, influenza, powdery mildew, viral gastroenteritis, scrapie, and chronic wasting disease.

The field of biological science in which very small and parasitic living things like bacteria, protists, viroids, and fungi are being studied using various microscopes is called microbiology.

The different microbiological branches are bacteriology, virology, parasitology, immunology, and phycology. The several microbiological applications are biodegradation, food production, bioremediation, biotechnology, and genetic engineering.

The microorganisms like bacteria and viruses that invade and directly attack the host cells and cause disease are called pathogens.

The several properties associated with pathogenic agents or microorganisms are unicellular organisms, prokaryotic or eukaryotic, toxin production, hijack nutrients, colonize in the host cell, and parasites.