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Infectious diseases

Infectious diseases are distinguished from other illnesses and disorders because they can be transmitted from someone who is ill either directly or indirectly to other individuals, who then develop the same infectious disease and are also able to pass it on.

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Non-communicable diseases (or NCDs)

Health problems that cannot be transmitted between individuals, such as heart disease, diabetes, lung cancer, arthritis and depression.

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Physical damage to the body caused by accidents or violence.

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Symptoms are sensations in the body that only the person who is unwell can experience (e.g. a headache, pain in the abdomen, blurred vision and nausea).

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Signs of a disease

The signs of a disease are indicators of illness that other people can observe (e.g. a runny nose and frequent sneezing).

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Chickenpox (Varicella)

Chickenpox is a viral infection that causes an itchy rash of spots allover the body and flu-like symptoms.
Chickenpox often starts without the classic rash, with a fever, headache, sore throat, or stomachache. These symptoms may last for a few days, with the fever in the 101°-102°F (38.3°-38.8°C) range.
The red, itchy skin rash usually starts on the abdomen or back and face, then spreads to almost everywhere else on the body(including the scalp, mouth, arms, legs, and genitals).
The rash begins as many small red bumps that look like pimples or insect bites. They appear in waves over 2 to 4 days, then develop into thin-walled blisters filled with fluid. The blister walls break,leaving open sores, which finally crust over to become dry, brown scabs.
All three stages of the chickenpox rash (red bumps, blisters, and scabs) appear on the body at the same time.

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Measles (Morbillo)

Measles is a highly contagious infection caused by the measles virus.
Initial signs and symptoms typically include fever, often greater than 40 °C (104.0 °F), cough, runny nose, and inflamed eyes.
2-3 days after the start of symptoms, small white spots may form inside the mouth, known as Koplik's spots.
3-5 days after the start of symptoms a red, flat rash which usually starts on the face and then spreads to the rest of the body would appear. Symptoms usually develop 10–12 days after exposure to an infected person and last 7–10 days.

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Koplik's spots

Clusters of tiny white spots inside the mouth (Could be a sign of measles).

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Identification of the underlying cause of an illness, based on its symptoms and signs, leading to a definitive disease name.

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More properly known as diabetes mellitus, a chronic disease that occurs when the pancreas does not produce enough insulin, or becomes insensitive to insulin. A common effect of uncontrolled diabetes is raised blood glucose (hyperglycaemia) which can gradually damage nerves and blood vessels.

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The respiratory organs that are located in the chest cavity; consisting of two elastic sacs with branching airways that allow air to be drawn into the body and expelled by a combination of muscular action and elastic recoil. They provide a large surface area where gaseous exchange occurs between the blood and the air.

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Where the body detects events happening in the environment.

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Acute condition

Short-term illness that develops rapidly and may be mild or severe, resulting in either recovery or death within a few days or weeks.

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Chronic condition

Long-term illness that develops slowly over months or years, resulting in progressively worsening symptoms unless treated.

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Organisms consisting of a single cell about 10 times smaller than a typical animal cell, containing ‘free’ genetic material (i.e. not enclosed in a nucleus). Singular, bacterium.

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Tuberculosis is an infectious disease usually caused by a bacteria, it generally affects the lungs, but can also affect other parts of the body.
The classic symptoms of active TB are a chronic cough with blood-containing sputum, fever, night sweats, and weight loss.

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Infectious agents: the parasites, protists, bacteria, fungi, viruses and prions that cause infectious diseases.
From the Greek word "pathos" (meaning to suffer) and "genès" (to produce).

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Infectious agents about 10 times smaller than bacteria and 100 times smaller than animal cells; they are not regarded as living because they don’t consist of cells, but resemble minute containers for one or more short strands of genetic material and a few other chemicals.

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Immune Sistem

A complex network of specialised cells, proteins and other natural substances that act together in a coordinated way to destroy pathogens that enter the body.

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An infected individual in whom pathogens are reproducing.

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In biology, transmission can refer to the passing of a disease-causing pathogen from one individual to another, or of nerve signals from one neuron to another.

In physics, transmission is the process in which light passes through a medium.

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Contagious Infection

When touch, such as a handshake, transfers pathogens to a susceptible person; they may enter the new host through a cut or graze, or be transferred from hand to mouth.

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Sexually transmitted infection (STI)

Infections resulting from direct person-to-person transmission of pathogens by genital, anal or oral sexual contact with an infected individual.

Involving infected semen, vaginal secretions, saliva or blood transmitting pathogens to the infected individual’s partner during unprotected sex.

Sexual transmission is more likely if the partner’s genitals, mouth or rectum are inflamed, for example, by another STI such as gonorrhoea or syphilis.

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Mother-to-child transmission

Direct transmission of pathogens from an infected mother to her unborn baby in the uterus, or during childbirth or breast feeding.

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The final part of the large intestine in which undigested solid waste (faeces) is stored before being expelled.

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Indirect person-to-person transmission

Indirect person-to-person transmission occurs when the original host sheds pathogens into the air, water, food or objects in the environment, which then infect someone else.

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Airborne infections

Most airborne infections are transmitted when a cough or sneeze expels fine droplets of water (known as an aerosol) containing millions of bacteria or viruses (Figure 1.8). The aerosol droplets may be inhaled by a susceptible person, or settle on surfaces where the pathogens contaminate hands, utensils, clothing, water or food, which are then touched or consumed by someone else.

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Waterborne infections

Infections resulting from indirect transmission of pathogens in contaminated water, or infection with pathogens that occur naturally in environmental water sources (e.g. cholera bacteria).

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Faecal–oral infections

Infections resulting from indirect transmission of faecal pathogens to the mouth on hands, food, objects, soil, etc. contaminated with faeces.
Flies can transfer pathogens from faeces to food via their feet.

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Foodborne infections

Infections resulting from indirect transmission of pathogens in food, or infection with pathogens originating in foodstuffs, such as raw meat and eggs, and unwashed fruits and vegetables.

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Any inanimate object contaminated with pathogens, e.g. clothing, door handles, cooking utensils, cups, medical equipment, etc.

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Bloodborne infections

Infections resulting from indirect transmission of pathogens in blood or blood products, either during medical procedures or when injecting drug-users share equipment.

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Infectious diseases that can be naturally transmitted to humans from other animals (usually vertebrates i.e. animals with backbones)
The influenza viruses that cause these diseases can sometimes be transmitted from animals to humans during the slaughter or handling of livestock.

A zoonosis that is well known in India and some other parts of the world is rabies.

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Vector-borne infections.

These infections are transmitted by an invertebrate animal (without a backbone), mainly biting insects and ticks.

The term ‘vector’comes from the Latin word for ‘carrier’.

The pathogen must complete part of its life cycle in the vector, so transmission to humans may be prevented if the vectors can be killed.

Many vector-borne infections are also regarded as zoonoses because the vector transmits the infection between vertebrate animals and humans : Lyme disease

Malaria for example is not regarded as zoonoses because the vector transmits the infection between humans (Bloodborne?)

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Biological compounds used as medical drugs to kill bacteria in the body.

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Injection or oral application of a vaccine containing killed or harmless pathogens, or substances derived from them; the aim is to generate a protective immune response in vaccinated individuals if they are later infected with the live pathogens on which the vaccine was based.

The term derives from the Latin vacca (cow), because the first vaccination in 1793 used matter from cowpox pustules to protect against smallpox. The modern term ‘immunisation’ is used interchangeably with vaccination.

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Emerging infectious diseases (EIDs)

the collective term for a group of conditions that pose new threats to human health. EIDs can be distinguished into three types:

1) Infectious diseases caused by previously unknown pathogens (e.g. HIV)

2) Diseases spreading far beyond their original geographical range (e.g. Ebola)

3) Rapidly resurging diseases due to antibiotic resistance or transmission in immunodeficient population groups (e.g. TB).

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EIDs 1) New infectious diseases caused by previously unknown pathogens

Many of the diseases in this category are caused by zoonotic viruses:

The virus that causes AIDS (HIV) may have originated in monkeys. (1984)

SARS (severe acute respiratory syndrome) which may have originated in poultry.

MERS (Middle East respiratory syndrome), which may have originated in camels.

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EIDs 2) Infectious diseases that have spread far outside their original range:

One example is Ebola virus disease (EVD), a ‘haemorrhagic’ fever, which means it causes severe internal bleeding.

Cases have occurred from time to time in remote villages in West Africa, but the first urban cases were detected in the capital of Guinea in February 2014 and from there Ebola quickly spread to neighbouring Liberia and Sierra Leone, folowed by Nigeria, Mali, Senegal and the USA.

By February 2015, over 9250 deaths had occurred from almost 14 000 laboratory-confirmed cases, so the ‘case fatality rate’ (the proportion of confirmed infected people who die) was around 58 per cent.

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EIDs 2) Infectious diseases that have spread far outside their original range

One example is Ebola virus disease (EVD), a ‘haemorrhagic’ fever, which means it causes severe internal bleeding.

Cases have occurred from time to time in remote villages in West Africa, but the first urban cases were detected in the capital of Guinea in February 2014 and from there Ebola quickly spread to neighbouring Liberia and Sierra Leone, folowed by Nigeria, Mali, Senegal and the USA.

By February 2015, over 9250 deaths had occurred from almost 14 000 laboratory-confirmed cases, so the ‘case fatality rate’ (the proportion of confirmed infected people who die) was around 58 per cent.

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EIDs 3) Previously declining infectious diseases that have resurged

Tuberculosis and some other infections caused by bacteria are a growing health concern because the causative pathogens are becoming increasingly resistant to antibiotics.

An additional factor in countries where HIV/AIDS is common is that infection with HIV suppresses the body’s immune defences, so people with AIDS are more susceptible to other infections, including TB.

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The scientific method

A method of procedure in scientific investigations consisting of systematic observation, measurement, experiment and the formulation, testing and modification of an hypothesis.

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A term describing a procedure conducted according to a fixed, pre-determined plan in which the methods, materials, conditions and sequence of actions are precisely defined.

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A statement proposing an explanation for an observation, made on the basis of limited evidence, as a starting point for further investigation or experiment.

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The infant mortality rate (IMR)

The number of deaths occurring under one year of age per 1000 live births in a population (‘per’ means ‘out of every’, so the IMR tells you how many out of every 1000 live-born babies died in infancy)

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Under-five child mortality rate

The number of deaths before the fifth birthday per 1000 live births in a population.

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Mortality rate

The number of deaths occurring per 1000 individuals in a population, or per 10 000, per 100 000, or per million population.

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Line Graph

A method of presenting numerical data plotted on a grid between vertical and horizontal axes, each marked with a scale. The data points are joined by lines that makes it easy to see how the pattern of values changes from left to right across the graph.

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Data Points

Numerical values represented by dots, crosses or other symbols plotted on a graph.

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Any value that varies in relation to the dimensions of another factor (which may also be a variable), e.g. a child’s height varies with his or her age, so height and age are variables in this example.

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The accurate, reproducible determination of ‘how much’ of something exists (e.g. its length, mass, volume, duration, speed, temperature, etc.).

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The statistical study of data on the occurrence, distribution, potential causes, prevention and control of diseases, disorders and disabilities in human populations.

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The statistical study of data on the number of births, deaths, marriages, size of each age group, the sex ratio, occupations, where people live, their education and income levels, etc.

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Refers to infectious diseases that may fluctuate over time but are always present in a population, e.g. the common cold.

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Cholera is a diarrhoeal disease caused by a specific type of bacteria, that occur naturally in environmental sources of water, such as rivers, lakes and estuaries.

Cholera was endemic (always present) in 19th-century England, as it is in many parts of the world today.

People infected with cholera bacteria produce large amounts of watery, foul-smelling, pale diarrhoea, which results in rapid dehydration and loss of essential salts from the body. Unless the fluids and salts are rapidly replaced, death follows in about one-third of cases within a few days.

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Medical ethics

Medical ethics is a set of principles that govern the way that doctors and other health professionals conduct their interactions with patients.

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An organism that lives in or on the body of another organism (its host) and benefits at the host’s expense; the term is generally only applied to pathogenic protists and worms, although strictly speaking all pathogens meet this definition.

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Neglected Tropical Diseases

A group of 17 infectious diseases mainly affecting populations in tropical regions (e.g. leprosy, rabies), identified by the World Health Organization as ‘neglected’ because despite their significant impact on health they attract relatively little media attention or research investment in prevention and treatment.

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Single-celled organisms that share the basic features of animal cells; pathogenic protists infect humans and cause diseases including malaria and cryptosporidiosis.

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A life form composed of one or more cells, which is capable of reproduction.

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Cell Membrane

The thin membrane forming the boundary of living cells, enclosing the cytosol and organelles; among its functions is the control of substances passing into and out of the cell.

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The watery fluid inside living cells that surrounds the organelles and other intracellular structures, in which many chemical reactions take place.

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The membrane-bound structures within a cell, such as the nucleus and mitochondria.

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The organelles in a cell in which chemical energy from nutrients is converted to a form that can be used by the cell. Singular, mitochondrion.

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The largest organelle in a cell enclosing the genetic material (DNA); bacterial cells differ from other cells in not having a nucleus.

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Deoxyribonucleic acid; large molecules present in almost all cells as the main constituent of chromosomes. DNA carries genetic information.

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Ectoparasites is the biological term for invertebrates (animals without backbones) that live on the surface (ektos is Greek for ‘outside’) of the human body, such as head lice, body lice and ticks.

Their bites can cause intense irritation and they also transmit some potentially life-threatening pathogens to humans, such as the bacteria that cause typhus.

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The 287 species of human parasites are all multicellular endoparasites that live inside the body (endon is Greek for ‘within’).

They can invade vital organs or live in the gut, bloodstream or tissues. The endoparasites of humans belong to four types of worms:

filarial (thread) worms
flukes (or flatworms).

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Commensal Bacteria

So-called ‘friendly’ bacteria living in or on the human body without causing harm and sometimes benefiting their host, e.g. some commensal bacteria in the gut help to digest plant-based foods.

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