An infection happens when a foreign organism enters a person’s body and causes harm. The organism uses that person’s body to sustain itself, reproduce, and colonize. These infectious organisms are known as pathogens. Examples of pathogens include bacteria, viruses, fungi, and prions. Pathogens can multiply and adapt quickly.
Some infections are mild and barely noticeable, but others are severe and life-threatening, and some are resistant to treatment. Infection can be transmitted in a variety of ways.
These include skin contact, bodily fluids, contact with feces, airborne particles, and touching an object that an infected person has also touched. How an infection spreads and its effect on the human body depend on the type of agent.
The immune system is an effective barrier against infectious agents, but colonies of pathogens may grow too large for the immune system to fight. At this stage, infections become harmful.
Many pathogens give off toxins that trigger negative responses from the body.
Types of Infection
Bacteria, viruses, fungi, protozoa, parasites, and prions are different types of pathogen. They vary in their size, shape, function, genetic content, and how they act on the body.
For example, viruses are smaller than bacteria, and they can enter a host and take over cells. However, bacteria can survive without a host.
Treatment will depend on the type of pathogen.
Viral infections are caused by a virus. Millions of types of virus are thought to exist, but only about 5,000 types have been identified. Viruses contain a small piece of genetic code. They are protected by a coat of protein and fat.
Viruses invade a host and attach themselves to a cell. As they enter the cell, they release genetic material. The genetic material forces the cell to replicate, and the virus multiplies. When the cell dies, it releases new viruses, and these go on to infect new cells.
Not all viruses destroy their host cell. Some of them change the function of the cell. In this way, viruses such as human papillomavirus (HPV) or Epstein-Barr virus (EBV) can lead to cancer by forcing cells to replicate in an uncontrolled way.
They can also target certain age groups, such as infants or young children.
A virus may remain dormant for a period before multiplying again. The person with the virus can appear to have recovered but may get sick again when the virus reactivates.
Bacteria are single-celled microorganisms known as prokaryotes.
There are estimated to be at least one nonillion bacteria on Earth. A nonillion is a one followed by 30 zeros. Much of Earth’s biomass is made up of bacteria.
Bacteria take three main shapes:
Spherical: These are usually the simplest to treat and are known as cocci.
Rod-shaped: These are called bacilli.
Spiral: Coiled bacteria are known as spirilla. If the coil of a spirillus is particularly tight, they are known as spirochetes.
Bacteria can live in almost any kind of environment, from extreme heat to intense cold, and some can even survive in radioactive waste.
There are trillions of strains of bacteria, and few of these cause diseases in humans. Some of them live inside the human body without causing harm, for example in the gut or airways. Some “good” bacteria attack “bad” bacteria and prevent them from causing sickness.
A fungus is an often multi-cellular parasite that can decompose and then absorb organic matter using an enzyme.
They almost always reproduce through the spreading of single-celled spores, and the structure of a fungus is normally long and cylindrical with small filaments branching from the main body. This structure is known as hypha.
There are approximately 51 million species of fungus.
Examples of fungal infections are:
valley fever, or coccidioidomycosis
some eye infections
A rash can be an indicator of a fungal infection of the skin.
A prion is a protein that contains no genetic material. It is normally harmless, but if it folds into an abnormal shape, it can become a rogue agent and affect the structure of the brain or other parts of the nervous system.
Prions do not replicate or feed on the host but trigger abnormal behavior in the body’s cells and proteins.
Prion diseases are rare, but they progress rapidly, and all are currently fatal.
Prions cause degenerative brain diseases, such as: bovine spongiform encephalopathy (BSE), also known as mad cow disease Creutzfeldt-Jakob disease (CJD)
While the forms of infection mentioned above are the main types, there are others that can have an effect on the body.
A single-celled organism with a nucleus can cause a protozoan infection. Protozoa commonly show features similar to animals, such as mobility, and can survive outside of the human body. They are most commonly transferred by contact with feces.
When they enter the human body, protozoa can also cause infection. Amebic dysentery is an example of a protozoan infection.
Helminths are larger, multicellular organisms that tend to be visible to the naked eye when full-grown. This type of parasite includes flatworms and roundworms. These are also able to infect the human body.
Finally, ectoparasites such as mites, ticks, lice, and fleas can cause infection by attaching or burrowing into the skin.
The term can also include blood-sucking arthropods, such as mosquitos, that transmit infection by consuming human blood.