introduction

Medical microbiology: the field of microbiology that deal with study of pathogenic microorganisms and their interactions with human as causative agents of infectious diseases. It covers the branches of bacteriology, virology, mycology, parasitology and immunology. While the microorganisms/microbes are microscopic living organisms which are very diverse and invisible by naked eye. They can be found in nearly every environment. The life in our planet is impossible without microbes due to their important role in generation of energy.
Types of microbial pathogens :
The pathogenic organisms of infectious diseases can be classified into five major groups of organisms : viruses, bacteria , fungi , parasites and prion.
? Viruses: are the smallest infectious agents (20 nm-300 nm). They are smallest 10,000 time than bacteria.Viruses are intact particles and a cellular entities. They contain one type of nucleic acid, either DNA or RNA surrounded by protein coat. They are replicate only within living cells
? Bacteria : are single cells, larger and more complex than viruses. They have cell wall and possess both types of nucleic acid (DNA,RNA), but lack true nucleus and organelles( prokaryotic cell).They are replicate by binary fission.
? Fungi: are larger than bacteria and have more advanced cell structures. Fungi have true nucleus (eukaryotic cell), cell wall and cellular organelles. They are involved two types of organisms; yeasts (unicellular) and molds (multicellular). They are replicate by budding or mitosis.
? Parasites: they are include two types: protozoa: are general term used to refer to part of parasites which is unicellular eukaryotic cells, while helminthes (metazoan) are more complex , multicellular eukaryotic cells.
? Prions: infectious particles that are composed of only protein. These proteins have ability to cause disease. The prion diseases are called spongiform encephalopathies include: Scrapie disease in sheep, Mad cow in cattle, Kuru disease in human .
Not all microorganisms are harmful for human, but most microorganisms are benefit