A pathogen is a generic term for any kind of agent that starts a disease in the organism. Pathogens can be bacteria, fungi, viruses or parasites. They can cause a wide range of disease, from mild ones like common cold to very severe infections such as meningitis or food poisoning. There are many ways that pathogens can enter the human body, for example swimming in contaminated water, eating infected or poorly cooked food and direct contact with body fluids. Pathogens in the water are some of the most dangerous and some countries impose strict measures to eliminate them. However, everyone can respect a few basic rules that offer protection from pathogens.
People transmit pathogens from one to another through direct contact. Many infectious agents are spread through saliva, which can travel airborne for a short distance. The most common such example is cold. Some of the most dangerous pathogens, like the Human Immunodeficiency Virus (HIV), can only be transmitted through contact with body fluids, such as sexual intercourse or infected needles. Feces and other organic waste materials can also spread various diseases in areas with very poor hygiene. This is why proper sanitation is essential to keep diseases in check.
Water is one of the main hosts for various organisms. Infested water is one of the most important causes of disease, especially in areas where fresh water is not treated. Most countries treat their water supply, which kills most of the pathogens in it and make the water safe to drink. However, contamination is always a risk even in the most advanced countries, due to either negligence or any disasters that can stop the water treatment process, like a powerful storm. When the water is unsafe or you have doubts that it has been treated properly, boiling it is an old and tested solution to eliminate most harmful agents.
Every pathogen can trigger one or more diseases in the body. For example, most cases of food poisoning are caused by an infestation with the E. coli bacteria, while a virus starts chicken pox. Fungi attack plants in particular but there are a number of human diseases like athlete's foot, oral thrush or ringworm that have a fungal origin.
A particular type of pathogen is Naegleria fowleri, which is actually an amoeba. It is widespread in nature and can be found in most water sources. Swimming in waters contaminated with it can be extremely dangerous, since it enters the nose and can travel up to the brain. It is the direct cause of a condition named primary amoebic meningoencephalitis (PAM), which kills off brain cells. It is a very rare type of infection and it usually has fatal consequences.
Viruses are very small agents that need a living organism in order to reproduce themselves. If they manage to penetrate into the human body, they start by infesting a cell then multiply and infect other nearby cells. Some viral diseases are mild, like the common flu, while others are chronic or even lethal conditions, like hepatitis or the human immunodeficiency virus (HIV).
Bacteria live everywhere, including inside the human body, and most types are harmless. However, some of them can cause serious diseases if they enter the organism. Bacteria also need alive human cells in order to feed and reproduce. Bacterial attacks on human cells can cause many diseases, from pneumonia to botulism or syphilis. Antibiotics are able to kill most bacterial infestations.
Fungi can cause a number of human diseases, such as ringworm, histoplasmosis and vaginal yeast infections. Examples of fungi pathogens include mushrooms, yeast and mold. Fungi cells consist of a nucleus and other parts protected by membranes, a type of cells known as eukaryotes. This makes them very resilient and a fungal infestation is very hard to kill. Anti-fungal drugs are not as effective as antibiotics and also have a lot of negative side effects.
Parasites are a type of pathogens that simply live inside a larger host and suck energy from it. They can sometimes be undetected but also cause diseases in some situations. Helminths, protozoa and ectoparasites are the tree main types of parasites that can infest humans. The most common parasitic diseases are malaria (caused by plasmodium), digestive issues (caused by tapeworms and other similar creatures) and Lyme disease (started by ticks).
Prions have only recently been included among pathogens. They are a type of infected proteins that can eventually disrupt the nervous system by concentrating inside the neural cells.
Protozoa are another example of single-cell pathogen organisms. They can cause infections but some of them also live as parasites and use the human body as a host. Protozoa are usually found in the food and can cause diseases such as amoebic dysentery.
Viroids are the smallest type of pathogens identified so far. They consist of just small pieces of RNA with a circular shape, without any protein layer. They are extremely small genetic agents, which usually affect plants. One example of a viroid disease is hepatitis D, which can be considered just a sequence of corrupted RNA. Viroids reproduce themselves through RNA polymerase II.