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Sherlock Holmes

on 16 May 2017

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Infectious diseases are disorders caused by organisms — such as bacteria, viruses, fungi or parasites. Many organisms live in and on our bodies. They're normally harmless or even helpful, but under certain conditions, some organisms may cause disease. Some infectious diseases can be inherited.
Disease causing agents are called pathogens. In biology, a pathogen in the oldest and broadest sense is anything that can produce disease; the term came into use in the 1880s. Typically the term is used to describe an infectious agent such as a virus, bacterium, protozoa, prion, a fungus, or other micro-organism.
Zika virus under microscope

It is spread by daytime-active Aedes mosquitoes.
Germ theory states that many diseases are caused by the presence and actions of specific micro-organisms within the body. The theory was developed and gained gradual acceptance in Europe and the United States from the middle 1800s. It eventually superseded existing miasma and contagion theories of disease and in so doing radically changed the practice of medicine. It remains a guiding theory that underlies contemporary biomedicine.
The atmospheric germ theory, from a lecture given to the Royal College of Surgeons in Edinburgh, 1868.
The evolution of the germ theory of disease can be traced to ancient speculations about the noxious entities that were thought to cause epidemic and endemic diseases. The Greek physician Hippocrates of Cos (c.460–375 BC), traditionally honored as the father of Western medicine, and his followers generally attributed infectious disease to poisonous vapors or “miasmas” that arose from swamps or putrid, decomposing materials. Health and disease depended on interactions between such atmospheric conditions and the four humors that made up the human body, a concept known as the atmospheric-miasmatic theory of disease. Other medical writers speculated that the noxious particles in disease-causing miasmas might actually be living entities. These mysterious little animals, seeds, worms, or “ferments” might be disseminated through the air or transmitted directly from the sick to others by means of contact (contagion).

EHEC bacteria under microscope
Some infectious diseases can be passed from person to person. Some are transmitted by bites from insects or animals. And others are acquired by ingesting contaminated food or water or being exposed to organisms in the environment.

Signs and symptoms vary depending on the organism causing the infection, but often include fever and fatigue. Mild infections may respond to rest and home remedies, while some life-threatening infections may require hospitalization.

Many infectious diseases, such as measles and chickenpox, can be prevented by vaccines. Frequent and thorough hand-washing also helps protect you from most infectious diseases.f
Each infectious disease has its own specific signs and symptoms.
General signs and symptoms common to a number of infectious diseases include:

Muscle aches
Hosts can fight infections using their immune system. Mammalian hosts react to infections with an innate response, often involving inflammation, followed by an adaptive response.

Specific medications used to treat infections include antibiotics, antivirals, antifungals, antiprotozoals, and antihelminthics. Infectious diseases resulted in 9.2 million deaths in 2013 (about 17% of all deaths). The branch of medicine that focuses on infections is referred to as infectious disease.
Symptomatic infections are apparent and clinical, whereas an infection that is active but does not produce noticeable symptoms may be called inapparent, silent, subclinical, or occult. An infection that is inactive or dormant is called a latent infection. An example of a latent bacterial infection is latent tuberculosis.

The word infection can denote any presence of a particular pathogen at all (no matter how little) but also is often used in a sense implying a clinically apparent infection (in other words, a case of infectious disease).

A short-term infection is an acute infection. A long-term infection is a chronic infection. Infections can be further classified by causative agent (bacterial, viral, fungal, parasitic), and by the presence or absence of systemic symptoms (sepsis).
Subclinical versus clinical (latent versus apparent)
One way of proving that a given disease is "infectious", is to satisfy Koch's postulates (first proposed by Robert Koch), which demands that the infectious agent be identified only in patients and not in healthy controls, and that patients who contract the agent also develop the disease.
Infectious diseases are sometimes called contagious disease when they are easily transmitted by contact with an ill person or their secretions (e.g., influenza). Thus, a contagious disease is a subset of infectious disease that is especially infective or easily transmitted.
By anatomical location: Urinary tract infection
Skin infection
Respiratory tract infection
Odontogenic infection (an infection that originates within a tooth or in the closely surrounding tissues)
Vaginal infections
Intra-amniotic infection
Koch's postulates is a set of criteria to be obeyed before it is established that a particular organism causes a particular disease. The organism must be present in every case and must be isolated, cultured and identified; it must produce the disease when a pure culture is given to susceptible animals; and it must be recoverable from the diseased animal. (Robert Koch, 1843–1910, German bacteriologist)
Koch's postulates:

1. The agent must be present in all cases of the disease

2. The agent must be isolated from someone with the disease and grown in pure culture

3. Inoculation into a susceptible organism of the agent–from a pure culture—must produce the disease

4. The agent must be recovered from the infected–inoculated organism and grown again in culture
Viruses are microscopic parasites, generally much smaller than bacteria. They lack the capacity to thrive and reproduce outside of a host body. Viruses teeter on the boundaries of what is considered life. On one hand, they contain the key elements that make up all living organisms: the nucleic acids, DNA or RNA (any given virus can only have one or the other). On the other hand, viruses lack the capacity to independently read and act upon the information contained within these nucleic acids. A minimal virus is a parasite that requires replication (making more copies of itself) in a host cell.
First, viruses need to access the inside of a host’s body. Respiratory passages and open wounds can act as gateways for viruses. Sometimes insects provide the mode of entry. Viruses will then attach themselves to host cell surfaces. They do so by recognizing and binding to cell surface receptors, like two interlocking puzzle pieces. Many different viruses can bind to the same receptor
and a single virus can bind different cell surface
receptors. Viral genomes direct host cells to
ultimately produce viral proteins (many a time
halting the synthesis of any RNA and proteins
that the host cell can use).
Many of us know bacteria only as “germs,” invisible creatures that can invade our bodies and make us sick.
Few know that many bacteria not only coexist with us all the time, but help us do an amazing array of useful things like make vitamins, break down some garbage, and even maintain our atmosphere.

Bacteria consist of only a single cell, but don't let their small size and seeming simplicity fool you. They're an amazingly complex and fascinating group of creatures. Bacteria have been found that can live in temperatures above the boiling point and in cold that would freeze your blood. They "eat" everything from sugar and starch to sunlight, sulfur and iron. There's even a species of bacteria—Deinococcus radiodurans—that can withstand blasts of radiation 1,000 times greater than would kill a human being.
Bacteria and archaea are the only prokaryotes. All other life forms are Eukaryotes (you-carry-oats), creatures whose cells have nuclei.
Like dinosaurs, bacteria left behind fossils. The big difference is that it takes a microscope to see them. And they are older.

arbovirus encephalitis
Human papillomavirus (HPV)
Herpetic whitlow
A protist is any eukaryotic organism that is not an animal, plant or fungus. The protists do not form a natural group, or clade, but are often grouped together for convenience, like algae or invertebrates. In some systems of biological classification, such as the popular 5-kingdom scheme proposed by Robert Whittaker in 1969, the protists make up a kingdom called Protista, composed of "organisms which are unicellular or unicellular-colonial and which form no tissues."
Some protists are significant parasites of animals (e.g., five species of the parasitic genus Plasmodium cause malaria in humans and many others cause similar diseases in other vertebrates), plants (the oomycete Phytophthora infestans causes late blight in potatoes) or even of other protists. Protist pathogens share many metabolic pathways with their eukaryotic hosts. This makes therapeutic target development extremely difficult – a drug that harms a protist parasite is also likely to harm its animal/plant host. A more thorough understanding of protist biology may allow these diseases to be treated more efficiently. For example, the apicoplast (a nonphotosynthetic chloroplast but essential to carry out important functions other than photosynthesis) present in apicomplexans provides an attractive target for treating diseases caused by dangerous pathogens such as plasmodium.
Worm classified as a parasite. (A parasite is a disease-causing organism that lives on or in a human or another animal and derives its nourishment from its host.) Lice are examples of parasites that live on humans; bacteria and viruses are examples of parasites that live either on humans or in humans; parasitic worms (also called helminths) live in humans.
The Kingdom Fungi includes some of the most important organisms, both in terms of their ecological and economic roles. By breaking down dead organic material, they continue the cycle of nutrients through ecosystems. In addition, most vascular plants could not grow without the symbiotic fungi, or mycorrhizae, that inhabit their roots and supply essential nutrients. Other fungi provide numerous drugs (such as penicillin and other antibiotics), foods like mushrooms, truffles and morels, and the bubbles in bread, champagne, and beer.

Fungi also cause a number of plant and animal diseases: in humans, ringworm, athlete's foot, and several more serious diseases are caused by fungi. Because fungi are more chemically and genetically similar to animals than other organisms, this makes fungal diseases very difficult to treat. Plant diseases caused by fungi include rusts, smuts, and leaf, root, and stem rots, and may cause severe damage to crops. However, a number of fungi, in particular the yeasts, are important "model organisms" for studying problems in genetics and molecular biology.
An easy way to catch most infectious diseases is by coming in contact with a person or animal who has the infection. Three ways infectious diseases can be spread through direct contact are:

Person to person: A common way for infectious diseases to spread is through the direct transfer of bacteria,
viruses or other germs from one person to another.
These germs can also spread through the exchange of body fluids from sexual contact. The person who passes
the germ may have no symptoms of the disease, but may simply be a carrier.

Animal to person: Being bitten or scratched by an infected animal — even a pet — can make you sick and, in
extreme circumstances, can be fatal.

Disease-causing organisms also can be passed by indirect contact. Many germs can
linger on an inanimate object, such as a tabletop, doorknob or faucet handle.

Insect bites: Some germs rely on insect carriers — such as mosquitoes, fleas, lice or ticks
— to move from host to host. These carriers are known as vectors. Mosquitoes can carry
the malaria parasite or West Nile virus, and deer ticks may carry the bacterium that
causes Lyme disease.

Food contamination: Another way disease-causing germs can infect you is through contaminated food and water. This mechanism of transmission allows germs to be spread to many people through a single source. E. coli, for example, is a bacterium present in or on certain foods — such as undercooked hamburger or unpasteurized fruit juice.
Antibiotics are grouped into "families" of similar types. Bacteria also are put together in groups of similar types, such as streptococcus or E. coli.

Certain types of bacteria are especially susceptible to particular classes of antibiotics.
Treatment can be targeted more precisely if your doctor knows what type of bacteria
you're fighting.

Antibiotics are usually reserved for bacterial infections, because these
types of drugs have no effect on illnesses caused by viruses. But sometimes
it's difficult to tell which type of germ is at work. For example, some types of
pneumonia are caused by viruses while others are caused by bacteria.

The overuse of antibiotics has resulted in several types of bacteria developing resistance to one or more varieties of antibiotics. This makes these bacteria much more difficult to treat.

Drugs have been developed to treat some, but not all, viruses. Examples include the
viruses that cause:

Hepatitis B
Hepatitis C

Some diseases, including malaria, are caused by tiny parasites. While there are drugs to treat these diseases, some varieties of parasites have developed resistance to the drugs.
botulism - is a rare and potentially fatal illness caused by a toxin, produced by the bacterium Clostridium botulinum.
dracunculiasis - a worm
bourbon virus through tick
helicobacter infection - linked to the development of duodenal ulcers and stomach cancer
chagas disease enlarged colon = viral
actinomycosis fungal infection
tularemia lesion - bacterial
leprosy - bacteria
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