Pathogens cause disease. A pathogen (disease-causing agent) is a microorganism that causes disease in its host (human, animals, plant, another microorganism)
Pathogens include bacteria, viruses, fungi, protozoa, and a newly discovered class of pathogens prions.
Pathogens invade from outside the body and there are several substrates and pathways wherein they invade a host. As a group they are accounted for a large portion of human diseases.
BACTERIA: Though most bacteria are harmless and beneficial, few bacteria are pathogens. These single-celled living organisms are dependent on living human cells for their energy supply. Thus they damage or kill the human cells and cause diseases like TONSILLITIS, PNEUMONIA, BOTULISM, SYPHILIS etc. Bacterial infections are often treated with antibiotics.
VIRUSES: Viruses are tiny infectious agents consisting of a small piece of genetic material either RNA or DNA and surrounded by a protein coat. They are acellular microorganisms (not composed of cells)
As they gain entry into living host cells the viral genetic material (DNA or RNA) enters the cell. Their protein coats are dissolved to incorporate the already released viral genetic material into the cell's genetic material. Some viruses release their genetic contents into the cells cytoplasm by merging their outer coat with the cell membrane.
Viruses cause diseases such as AIDS, HEPATITIS, CHICKEN POX, ENCEPHALITIS, COLDS, FLU, MEASLES, RABIES, POLIO etc.
PRIONS: Prions refers to infectious proteins. Prion is an entirely new class of infectious pathogen. Caused by proteinaceous infectious particles, a common feature of this disease is the presence of unusual accumulations of misfolded proteins in our nerve cells, as they chock-off nerve cell functions. In other words the 'rogue proteins' refold normal prion proteins into the new shape causing the normal prion protein to lose its original function.
CREUTZFELDT-JAKOB disease is caused by Prions