> See also: > - [[Microbiomes]] # Pathogens A pathogen is any organism with the potential to cause disease. If an organism is not causing harm to the host organism it is often called a **commensal microorganism** and is part of that organism's microbiome **Opportunistic pathogens** can cause diseases if the host body's defenses are weakened or the pathogen makes its way into a part of the body it isn't normally found. ## Diversity of Pathogens There is a *constant evolutionary struggle* between pathogens and the host organism. Due to the rapid replication times of pathogens, they often have the upper hand. A **reservoir** is the natural environment in which the pathogen normally resides There are 4 types of pathogens that primarily affect humans: - Bacteria - Viruses - Fungi - Parasites (Unicellular protozoa, Multicellular worms) **Zoonoses** are diseases which ensue due to the transmision of infectious agents from animals to humans (Ex: Rabies) A **vector** is an organism that can transmit a pathogen from one host to another (Ex: mosquitoes, ticks and many other arthropods) A **fomite** is an *inanimate vehicle* (Ex: surgical instrument, bedding, drinking cup) that can *spread a pathogen* to multiple hosts **Intoxications** are diseases that can occur if the associated pathogen *does not need to be present* for the host to acquire it ### Intracellular vs Extracellular Pathogens > [!info]- Intracellular vs Extracellular Pathogens > > ![[Intracellular vs Extracellular Pathogens.png|500]] > > **Extracellular:** Those that live outside cells in extracellular spaces, on the surface of [[Histology|epithelia]], or in the [[Blood Cells|blood]] > - aaa > > **Intracellular :** Pathogens that can replicate inside human cells > - aaa > > **Facultative intracellular pathogens** can reside *within the host cells, in the environment, or even in pure cultures* without host-cell support (Ex: Brucella abortus, Histoplasma capsulatum). In contrast, **obligate intracellular pathogens** are *incapable of growth and multiplication outside a host* cell. - All viruses fall under this category and many bacterial pathogens (Ex: Chlamydia spp. & richettsias) ## The Course of Infectious Disease ![[Pasted image 20231126020144.png|300]] the pathogen has entered the host and is multiplying, but hasn't reached high enough levels to cause clinical manifestations ## Mechanisms of Action Because viruses do not have any method of producing their own energy, their mechanism of attacking the host always involves the “hijacking” of the host’s cellular machinery to aid the virus to further proliferate. - Ex: HIV relies on the chemokine receptors of certain immune cells