Infections are caused by tiny living things, called micro organisms, that get into the body, or onto anything with life and start multiplying. Some organisms are harmless and help our bodies to work properly. But others cause disease.
Infectious micro organisms are in many shapes and sizes. Bacteria and protozoans are microscopic one-celled organisms, while viruses are even smaller. Fungi grow like plants.
is the invasion of an organism's body tissues by disease-causing agents, their multiplication, and the reaction of host tissues to these organisms and the toxins they produce.
, are also known as transmissible disease
or communicable disease
, as the host succumbs to some sort of illness resulting from an infection.
Infectious disease can involve bacteria, fungi, parasites, or viruses, all of which are microorganisms. Microorganisms are present everywhere. Despite their overwhelming abundance, relatively few of the thousands of species of microorganisms invade, multiply, and cause disease in people.
Of those that do cause disease in people, some are highly communicable, or to say, easily transmitted among people. Many microorganisms live on the skin and in the mouth, upper airways, intestine, and genitals (particularly the vagina) without causing disease.
Whether a microorganism lives as a harmless companion to a person or invades and causes disease depends on the nature of the microorganism and on the state of the person's immunity levels or natural defenses.
Disease can arise if the host's protective immune mechanisms are compromised and the organism inflicts damage on the host. Microorganisms can cause tissue damage by releasing a variety of toxins or destructive enzymes.
Some Infections that occur are persistent, because the body is unable to clear the organism after the initial infection. Persistent infections are characterized by the continual presence of the infectious organism, often as latent infection with occasional recurrent relapses of active infection.
There are some viruses that can maintain a persistent infection by infecting different cells of the body. Some viruses once acquired never leave the body. A typical example is the herpes virus, which tends to hide in nerves and become reactivated when specific circumstances arise.
These kinds of persistent infections cause millions of deaths globally each year. Chronic infections by parasites account for a high morbidity and mortality in many developing and underdeveloped countries.
For infecting organisms to survive and repeat the infection cycle in other hosts, they or their progen, must leave an existing reservoir and cause infection elsewhere.
Infection transmission can take place through many potential routes, such as:
- Droplet contact, also known as the respiratory route, and the resultant infection can be termed as an airborne disease.
- If an infected person coughs or sneezes, the microorganisms, are suspended in the air, in warmth, as moist droplets, and may enter the body through the nose, mouth or eye surfaces of any other person, coming in contact with it.
- Fecal-oral transmission, wherein foodstuffs or water become contaminated (by people not washing their hands before preparing food, or untreated sewage being released into a drinking water supply) and the people who eat and drink them become infected.
- Common fecal-oral transmitted pathogens include Vibrio cholerae,Giardia species, rotaviruses,Entameba histolytica, Escherichia coli , and many more. Most of these pathogens cause gastroenteritis.
- Sexual transmission, with the resulting disease being called sexually transmitted disease
- Oral transmission, Diseases that are transmitted primarily by oral means may be caught through direct oral contact such as kissing, or by indirect contact such as by sharing a drinking glass or a cigarette, etc.
- Transmission by direct contact, Some diseases that are transmissible by direct contact include athlete's foot, impetigo and warts
- Vehicle Transmission, transmission by an inanimate reservoir, such as food, water, soil, etc.
- Vertical transmission, directly from the mother to an embryo, fetus or baby during pregnancy or childbirth. It can occur when the mother gets an infection as an intercurrent disease in pregnancy.
- Iatrogenic transmission, due to medical procedures such as injection or transplantation of infected material.
- Vector-borne transmission, transmitted by a vectors, which is an organism that does not cause disease itself, but that transmits infection by conveying pathogens from one host to another.
The relationship between virulence and transmissibility is complex; if a disease is rapidly fatal, the host may die before the microbe can be passed along to another host.
In most cases, microorganisms live in harmony with their hosts via mutual or commensal interactions. Diseases can emerge when existing parasites become pathogenic or when new pathogenic parasites enter a new host.
Human activity is involved with many emerging infectious diseases, such as environmental change enabling a parasite to occupy new niches. When that happens, a pathogen that had been confined to a remote habitat has a wider distribution and possibly a new host organism.
Parasites jumping from nonhuman to human hosts are known as zoonoses. Under disease invasion, when a parasite invades a new host species, it may become pathogenic in the new host.
Several human activities have led to the emergence of zoonotic human pathogens, including viruses, bacteria, protozoa, and rickettsia, and spread of vector-borne diseases.
- Bacteria are microscopic, single-celled organisms. There are thousands of different kinds, and they live in every conceivable environment all over the world. They live in soil, seawater, and deep within the earth’s crust. Some bacteria have been reported even to live in radioactive waste. Many bacteria live in the bodies of people and animals, on the skin and in the airways, mouth, and digestive, reproductive, and urinary tracts, without causing any harm. Such bacteria are called resident flora , or the microbiome. Many resident flora are actually helpful to people, for example by helping them digest food or by preventing the growth of other, more dangerous bacteria.
- A virus is a small infectious organism, much smaller than a fungus or bacterium, that must invade a living cell to reproduce or replicate. The virus attaches to a cell, called the host cell, enters it, and releases its DNA or RNA inside the cell. The virus’s DNA or RNA is the genetic material containing the information needed to make copies of, or replicate the virus. The virus’s genetic material takes control of the cell and forces it to replicate the virus. The infected cell usually dies because the virus keeps it from performing its normal functions. When it dies, the cell releases new viruses, which go on to infect other cells.
- Fungi are neither plants nor animals. They were once thought to be plants but are now classified as their own. Because fungal spores are often present in the air or in the soil, fungal infections usually begin in the lungs or on the skin. Fungal infections are rarely serious unless the immune system is weakened, usually by drugs or disorders. Fungal infections usually progress slowly. Fungi can grow in two forms: one as Yeasts, which are Single round cells, and the other as, Molds, in which many cells forming long, thin threads and are called hyphae. Some fungi go through both forms during their life cycle. Fungi often grow in soil and decaying plant material. Many fungi, including bread molds and mushrooms, can be seen with the naked eye. Fungi reproduce by spreading microscopic spores. These spores are often present in the air and soil, where they can be inhaled or come into contact with the surfaces of the body, primarily the skin. Consequently, fungal infections usually begin in the lungs or on the skin.
BIOLOGY OF AN INFECTIOUS DISEASE
- Microorganisms are tiny living creatures, such as bacteria, fungi and viruses. Microorganisms are present everywhere. Despite their overwhelming abundance, relatively few of the thousands of species of microorganisms invade, multiply, and cause disease in people.
Except for some superficial skin infections, fungal infections are rarely passed from one person to another.
Bacterial and viral infections can both cause the same kinds of symptoms. It can be difficult to distinguish which is the cause of a specific infection. It is important to distinguish, because viral infections cannot be cured by antibiotics.
There are a variety of infectious diseases and many can be highly contagious. Whilst some of these conditions are uncomfortable but short-lived, others can be chronic or morbid and sometimes also require lifelong management.