What is infection? what causes infections?

What is infection? what causes infections?

A human with an infection has another organism inside them which gets its sustenance (nourishment) from that person, it colonizes that person and reproduces inside them. The human with that organism (germ) inside is called the host, while the germ or pathogen is referred to as a parasitic organism. Another name for an organism that causes infection is an infectious agent.

It is only an infection if the colonization harms the host. It uses the host to feed on and multiply at the expense of the host to such an extent that his/her health is affected. The normal growth of the bacterial flora in the intestine is not an infection, because the bacteria are not harming the host.

An organism which colonizes and harms a host's health is often called a pathogen. Examples include:

  • Parasites
  • Fungi
  • Bacteria
  • Prions
  • Viroids (plant pathogens, they affect the health of plants)
We all develop a wide range of infections, but fight them off rapidly. Some people, however, develop persistent, long-term (chronic) infections. The majority of chronic infections are caused by viruses, such as hepatitis or herpes. Chronic bacterial infections are more likely to affected patients with diabetes, as well as those with weakened immune systems.

Sometimes, if two organisms are present in the host together, they fight each other instead of the human body, and the levels of each colony remain balanced - their presence, together does not pose a problem for the host. An example could be some skin bacteria and yeast. Antibiotics may, in fact, upset the balance by destroying the good bacteria, allowing the other potential pathogen to multiply faster and cause health problems.

Viral infections

Infections caused by a virus. An individual may become infect by:
  • Inhaling the virus (breathing it in)
  • Being bitten by infected insects or parasites
  • Through sexual contact
Respiratory infections of the upper airways, nose and throat are the most common forms of viral infections.

Some antiviral medications may help, they either undermine the virus' ability to reproduce, or boost the patient's immune system.

Viruses are tiny organisms, much smaller than bacteria or fungi. The virus invades its host and attaches to a cell, entering it and releasing genetic material (DNA or RNA). This genetic material helps the virus multiply; it takes over control of the cell, making it replicate the virus. A cell that has this genetic material inserted into it cannot function properly and soon dies. When it does it releases new viruses, which infect new cells, etc.

Not all viruses destroy their host cell. Some of them just alter what the cell does. Experts say that some cells become cancerous as a result of a virus interfering with its functions.

Sometimes the genetic material lies dormant in a cell; some time in the future something triggers the cell and the virus starts multiplying again, making the host ill.

Viruses target specific cells in the body, such as those in the genitals or upper respiratory tract. Some target certain age groups, such as babies or young children, such as those that cause croup. The rabies virus targets the cells in the host's nervous system. Viruses may target skin cells and cause warts.

However, some viral infections can be systemic - they affect many different parts of the body, causing for example runny nose, sinus congestion, cough, and body aches. A viral infection that causes, for example viral conjunctivitis is local. Viral infections that cause pain, often trigger itching or burning.

Bacterial infections

Bacteria are tiny single-cell microorganisms, usually a few micrometers in length that normally exist together in millions - they are neither plants nor animals - they belong to a group all by themselves. A gram of soil typically contains about 40 million bacterial cells. A milliliter of fresh water usually holds about one million bacterial cells.

Planet Earth is estimated to hold at least 5 nonillion bacteria. Scientists say that much of Earth's biomass is made up of bacteria.

5 nonillion = 5,000,000,000,000,000,000,000,000,000,000 (or 5x1030).

Bacteria come in three main shapes:

  • Spherical (like a ball)

    These are usually the simplest ones. Bacteria shaped like this are called cocci (singular coccus).

  • Rod shaped

    These are known as bacilli (singular bacillus).

    Some of the rod-shaped bacteria are curved; these are known as vibrio.

  • Spiral

    These known are as spirilla (singular spirillus). If their coil is very tight they are known as spirochetes.

A bacterial cell differs somewhat from the cell of a plant or animal. Bacterial cells have no nucleus and other organelles (sub-units within a cell with a specific function) bound by a membrane, except for ribosomes. Bacteria have pili, flagella, and a cell capsule (most of them), unlike animal or plant cells. An organism without a nucleus is called a prokaryote. (Click here for a more comprehensive article about bacteria)

Bacteria can live in virtually any kind of environment, from extremely hot to super cold, some even in radioactive waste. A number of bacteria live in our bodies, on our skin, airway, mouth, digestive and urinary tracts - most of the time without causing any harm to the host.

A relatively small number of bacteria causes diseases in humans. Some of the most deadly diseases and devastating epidemics in human history have been caused by bacteria, including:

  • Cholera
  • Diphtheria
  • Dysentery
  • Plague
  • Pneumonia
  • Tuberculosis
  • Typhoid
  • Typhus
Here is a list of infections and the names of the bacterium that commonly cause them (Source: Wikimedia Commons):
  • Brain (bacterial meningitis) - Streptococcus pneumoniae, Neisseria, meningitides, Haemuphilus influenzae, Streptococcus agalictaiae, Listeria monocytogenes.
  • Ear (otitis media ) - Streptococcus pneumoniae
  • Pneumonia

    Commonly acquired - Streptococcus pneumoniae, Haemophilus influenzae, Staphylococcus aureus.

    Atypical - Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella pneumophila

    Tuberculosis - Mycobacterium tuberculosis

  • Upper respiratory tract infection - Streptococcus pyogenes, Haemophilus influenzae
  • Gastritis (inflammation of the stomach) - Helicobacter pylori
  • Food poisoning - Campylobacter jejuni, Salmonella, Shigella, Clostridium, Staphylococcus aureus, Escherichia coli
  • Eye infections - Staphylococcus aureus, Neisseria gonorrheae, Chlamydia trachomatis
  • Sinusitis - Streptococcus pneumoniae, Haemophilus influenzae
  • Urinary tract infections - Escherichia coli, other Enterobacteriaceae, Staphylococcus saprophyticus, Pseudomonas aeruginosa
  • Skin infections - Staphylococcus aureus, Streptococcus pyogenes, Pseudomonas aeruginosa
  • STDs (sexually transmitted diseases) - Chlamydia trahomatis, Neisseria gonorrhoeae, treponema pallidum, Ureaplasma urealyticum, Haemophilus ducreyi
Bacterial infections typically present with localized redness and heat, inflammation (swelling) and pain. Bacterial infections tend to present localized pain more frequently than viral infections (not always). Even with a throat infection, a bacterial one will usually have more severe pain on one side of the throat. If there is pain in just one ear, it is more likely to be a bacterial infection.

Mycosis (fungal infection/disease)

Mycosis is a fungal infection in or on a part of the body, or a disease caused by a fungus. Some fungi reproduce through very small airborne spores which people either inhale or pick up on their skin - i.e. must fungal infections start in the lungs or the skin.

Patients on long-term strong antibiotics are at higher-than-normal risk of developing a fungal infection. Strong antibiotics can eventually reduce the population of good bacteria which help maintain the balance of microorganisms in the intestines, mouth, vagina and other parts of the body. If enough of these good bacteria are destroyed, the fungi have an opportunity to grow and cause health problems for the host.

Patients with weakened immune systems, such as those with HIV/AIDS and individuals receiving chemotherapy have a higher risk of developing a fungal infection. Diabetes patients, especially those who do not have good disease control are also more susceptible.

There are three types of fungal infections of the skin:

  • Superficial mycoses - limited to the surface of the skin and hair, such as Tinea versicolor, which commonly affects young people. The chest, back, upper arms or legs may be affected (very rarely the face). Light or reddish-brown spots appear on the skin. Sometimes the spots are not visible.
  • Cutaneous mycoses - occurs deeper in the skin, in the epidermis. The hair and nails may also be affected. Cutaneous mycoses are limited to the keratinized layers of skin, nails and hair. This type of mycosis is caused by dermatophytes (a group of three types of fungus that commonly causes skin disease in animals and humans), which may cause ringworm. Examples of dermatophytes are Microsporum, Trichophyton, and Epidermophyton fungi. Athlete's foot is another example of cutaneous micosis.
  • Subcutaneous mycoses - these types of infections go deeper into the skin, including the dermis, subcutaneous tissue, as well as muscle and fascia. The fascia is a band of tissue below the skin that covers underlying tissues - it separates different tissue layers and surrounds muscles. Subcutaneous mycoses tend to be long term (chronic) and are usually caused by skin penetration.
Systemic mycoses due to primary or opportunistic pathogens - a primary pathogen causes disease because of its very presence in a healthy human, while an opportunistic pathogen causes disease in a host (human) with a weakened immune system. Systemic means the entire body is affected.

Systemic mycoses due to primary pathogens tend to originate in the lungs, and then spread to other parts of the body; they are usually virulent (potent).

Candidiasis (thrush) is an example of systemic mycoses due to opportunistic pathogens; in other words, a patient is more likely to get thrush when their defenses (immune system) are down.

Prion disease

A prion is an infectious agent consisting mainly of protein - it contains no genetic material. It is neither bacterial nor fungal. It occurs normally in a harmless form, but when it folds into an abnormal shape it turns into a rogue agent and affects the structure of the brain or other parts of the nervous system. All forms of prion infections are currently untreatable and fatal.

Prions cause degenerative brain diseases, such as mad cow disease, CJD (Creutzfeldt-Jacob disease), kuru, fatal familial insomnia, and scrapie. Experts also link some cases of Alzheimer's disease to prion infection.

Prion diseases, also called transmissible spongiform encephalopathies, are brain wasting diseases that affect humans and animals. Made primarily of protein, prions are small active agents that act a bit like viruses: they don't replicate themselves but hijack material in the host and cause it to behave abnormally, for instance they force host proteins to fold into shapes that clump together into plaques that clog up the brain, causing it gradually to waste away.

A team of scientists from the US and the UK have found a new type of prion disease in mice that damages brain arteries and may help us better understand and treat types of Alzheimer's disease that cause similar damage. (link to article).

Worldwide deaths from infectious diseases

Source: World Health Organization (WHO)
  • Lower respiratory infections

    Deaths 2002 - 3.9 million

    Percentage of all deaths - 6.9% 4.1 million


    Deaths 2002 - 2.8 million

    Percentage of all deaths - 4.9%

  • Diarrheal diseases

    Deaths 2002 - 1.8 million

    Percentage of all deaths - 3.2%

  • Tuberculosis (TB)

    Deaths 2002 - 1.6 million

    Percentage of all deaths - 2.7%

  • Malaria

    Deaths 2002 - 1.3 million

    Percentage of all deaths - 2.2%

  • Measles

    Deaths 2002 - 0.6 million

    Percentage of all deaths - 1.1%

  • Pertussis (whooping cough)

    Deaths 2002 - 0.29 million

    Percentage of all deaths - 0.5%

  • Tetanus

    Deaths 2002 - 0.21 million

    Percentage of all deaths - 0.4%

  • Meningitis

    Deaths 2002 - 0.17 million

    Percentage of all deaths - 0.3%

  • Syphilis

    Deaths 2002 - 0.16 million

    Percentage of all deaths - 0.3%

  • Hepatitis B

    Deaths 2002 - 0.10 million

    Percentage of all deaths - 0.2%

  • Tropical diseases

    Deaths 2002 - 0.13 million

    Percentage of all deaths - 0.2%

Lung Infections - Causes And Types (Video Medical And Professional 2020).

Section Issues On Medicine: Disease