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Infectious Diseases Resources

What Are Infectious Diseases?

Infectious diseases (also known as transmissible diseases or communicable diseases) are clinically evident illnesses caused by microorganisms, such as bacteria, viruses, fungi, or parasites that can be spread – directly or indirectly – from one vector to another. Some infectious diseases are caused by person-to-person contact, others are passed via animal or insect bite, and still others are caused by ingestion of infected, contaminated food, water, or other environmental exposures.

Infectious diseases can even be caused by normally-inert microorganisms present on the skin or in our bodies that, under the right circumstances, can cause illness. These are called “opportunistic infections.”

Many infectious diseases, such as the chicken pox virus, can be prevented by vaccination; other infectious diseases can be prevented (or risk for infection lowered) by frequent and proper hand-washing.

Symptoms of infectious diseases may vary wildly based upon the microorganism that causes the disease, but many infectious diseases are accompanied by fever and chills. People may complain of mild complaints while others experience life-threatening infections that require IV antibiotics and hospital care.

Infectious diseases are often called “contagious diseases,” when these illnesses are easily transmissible from secretions or other contacts with an ill person. A contagious disease is easily transmitted and considered especially infective.

Not all infectious diseases are easily transmittable from person to person – many require other ways of transmission, such as via vector or sexual contact. Those who have less communicable diseases don’t require medical isolation or special precautions as the infectious agent is only transmissible via certain routes.

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How Are Microorganisms Classified?

While there exists an almost infinite amount of microorganisms, most do not cause disease. Infectious diseases are the result of the interplay between the few disease-causing pathogens and the host’s defenses. The severity and appearance of disease from a pathogen depends largely upon the ability of the pathogen to infect the host as well as the ability of the host to resist infection by the pathogen. Most clinicians classify infectious microorganisms according to the status of the host’s defenses:

1) Primary Pathogens cause disease due to their presence in a normal and healthy host. The virulence (the severity of the diseases they cause) of a microorganism is a consequence of the need for that microorganism to breed and spread. Most of the common primary pathogens that affect humans only infect humans, although it’s important to note that many of the serious diseases are caused by environmental factors. Primary pathogens often cause a more severe form of disease in those who already have decreased immune system or resistance.

2) Opportunistic Pathogens are microorganisms that infect a person with weakened defenses; often caused by microorganisms that normally in contact with the host without causing disease, like fungi or pathogenic bacteria in the GI tract. These impairments in the hosts defenses may be the result of genetic defects, immunosuppressive medications, radiation, or as a result of a disease that causes immunosuppression (such as HIV/AIDS)

How Are Infectious Diseases Transmitted?

Infectious diseases are transmitted from source to source, and learning about the mode of transmission is a vital component to understanding the inner workings of the microorganisms.

Respiratory Illnesses/Meningitis: caused by contact with aerosolized droplets caused by sneezing, coughing, kissing, laughing, talking, or singing.

GI Diseases: often spread by ingesting contaminated food or water.

Sexually Transmitted Diseases: spread through contact with bodily fluid, typically as the result of sexual activity.

Fomite: an inanimate object that, when in contact with it, can spread disease, such as a coin or a used tissue.

Vector: many diseases are carried through vectors, in which an agent picks up an infectious agent and transmits it to the host directly (through a mosquito bite, for example) or indirectly (like a fly landing on food).

What Are Microorganisms?

Most people tend to associate the word, “microorganism” with pathogen (or disease-producing), while, in reality, only a small subset of microorganisms cause disease.

Most microorganisms are crucial to the welfare of the world, by helping to maintain the balance of living organisms. Some examples of non-pathogenic microorganisms and their functions include:

  • Microorganisms found in bodies of water are the basis of the food chain in oceans, lakes, and rivers.
  • Soil Microorganisms help to break down wastes and incorporate nitrogen gas from the air into organic compounds, thereby recycling chemical elements.
  • Microbes play a role in photosynthesis – a food and oxygen process critical to life on earth.
  • The microbes within our body help digest our food as well as synthesize vitamins that our bodies require, such as Vitamin K and Vitamin B.

It’s now understood that microorganisms are found almost everywhere, which helps scientists and researchers determine the disease-causing organisms and develop methods to cure them.

How Are Microorganisms Named?

Nomenclature (naming) of organisms are traditionally done in Latin – the language of scholars. In this way, organisms are assigned two names. The genus, which is always capitalized, followed by the species name, which is never capitalized. Both names are either underlined or italicized.

After mention of an organism, the first initial (rather than the whole name) can be used following the species name. In this way, we’d describe humans as Homo sapiens during one part of a resource page while we could simply abbreviate it later by saying, H. sapiens.

What Are Types of Microorganisms?

There are a number of types of microorganisms. Each type of microorganism will be discussed in greater detail below, including common characteristics of each type the of microorganisms.

Bacteria As Infectious Diseases:

Bacteria (singular: bacterium) are simple, single-celled organisms that are not enclosed by a nuclear membrane, which makes bacterial cells “prokaryotics.”

While bacteria are often thought to be harmful, invisible microorganisms, less than one percent cause disease. In fact, most bacteria that live in the body aid in bodily functions such as digesting food, destroying other disease-causing microorganisms, as well as providing nutrients to the body. There even exist bacteria that help in the creation of cheese and yogurt.

Most bacterial cells appear in one of several forms:

  • Bacillus – rod-like bacterium
  • Coccus – oval-shaped bacterium
  • Spiral – curved and/or corkscrew shaped

Bacteria can move using a large variety of mechanisms:

Flagella – semi-rigid cylindrical type of appendages that function like a propeller on a boat. Flagella fall into a number of different arrangements on bacteria:

  • Monotrichous – a single flagella attached to the bacteria.
  • Lophotrichous – clusters of flagella at the pores of the cell.
  • Peritrichious – flagella are distributed evenly across the surface of the cell.
  • Spirochetes – unique type of bacteria have flagella between the two membranes in the periplasmic space.

Bacterial Gliding – movement of bacteria across the surfaces.

Twitching Motility – bacteria use their pili as a grappling hook, anchoring it, then retracting it.

It’s important to note that bacteria that cause infections can make one very ill, as they multiply quickly inside the body. Many of these infectious bacteria cause tissue damage by releasing toxins into the body. Examples of infectious bacteria include:

  • Staphylococcus
  • Streptococcus
  • Cholera
  • Syphilis
  • Anthrax
  • Black Plague
  • Escherichia coli

Most types of bacterial infections are managed by antibiotics, which can help to fend off the illness-causing bacteria and allow the person to return to a state of health.

Unfortunately, antibiotic use and overuse mean that there are antibiotic-resistant strains of bacteria that cannot be treated by normal antibiotics.

Bacteriophages are viruses that can infect bacteria – they infect and lyse their host bacteria, or they insert themselves into the bacterial chromosome – common examples include Escherichia coli O157:H7 and Clostridium botulinum.

Viruses As Infectious Diseases:

Viruses (singular virion) are tiny microorganisms that require the use of an electron microscope to visualize. Structurally simple, viruses are not cellular (acellular), instead the virus particle contains a core surrounded by a protein coat (sometimes covered in a second layer – a lipid membrane, called an envelope) made simply of one type of nucleic acid: DNA or RNA.

All living cells have RNA and DNA, that carry out chemical reactions and reproduce as self-sufficient units. Viruses can only replicate by using the cellular makeup of other organisms. This means that viruses are actually parasitic and can attack animals, plants, bacteria, and humans.

A virion is comprised of three parts:

  1. Nucleic Acid – forms the core of the virus with DNA or RNA, which holds all of the information about the cell and aids in virus replication.
  2. Capsid (Protein Coat) – this covering protects the nucleic acid.
  3. Envelope (Lipid Membrane) – covers the capsid of the virus cell; although not all viruses have an envelope – these are called “naked viruses.”

Viruses are spread in many ways:

  • By vectors – disease-bearing organisms that infect other organisms.
  • Coughing and sneezing – as is the case with the influenza virus
  • Fecal-oral route of transmission – via contact or food and water.
  • Sexual contact – such is the case with HIV.

Prevention of viral infection can occasionally be done utilizing vaccines – artificially acquired immunity to a specific virus. There are a number of viruses that evade our immune response and lead to chronic illness.

Antibiotics have zero effect on viruses, but several drugs called “antivirals” have been developed.

Replication cycle of a virus, which are acellular organisms that use a host cell to produce copies of themselves and assemble themselves within a cell. The replication cycle of a virus includes the following six stages:

  1. Attachment – a specific binding between receptors on the hosts cellular surface and the viral capsid proteins. Viruses have evolved to favor viruses that only infect cells that are capable of replication.
  2. Penetration – viruses enter the host cell in a process known as “viral entry.”
  3. Uncoating – the removal of the viral capsid by degradation of enzymes, and the end result is the release of the viral nucleic acid (DNA or RNA).
  4. Replication – involves multiplication of the genome and assembly of the viral proteins.
  5. Assembly – the viral particles are assembled, either within the host cell or once outside of the host cell.
  6. Release – Lysis (process of killing the cell wall and membrane – if present) occurs and the virus is released into the body.

Specific Viruses Include the following:

  • HIV
  • Common cold
  • Warts
  • Smallpox
  • Influenza

It’s important to remember that viruses, unlike other microorganisms are not alive, cannot grow or multiply on their own, and require a host cell for replication. Viruses may also infect bacterial cells.

Virus particles are extraordinarily small; up to 100 times smaller than a single bacterium, which is ten times smaller than a human cell.

Fungi As Human Pathogens:

While fungal diseases sound mysterious and strange, they’re often caused by fungi common in the soil, on plants, in trees, on our skin, inside our intestinal tracts, and on our mucous membranes. In fact, most fungi are not only not harmful, they’re helpful. Pennicilin, one of the most important antibiotics is made from fungi, as are many different types of alcoholic beverages.

However, there are certain types of fungi that can be pathogenic and cause infectious disease in humans.

Symptoms of a fungal infection depend wildly upon the type of infection and location within the body. Fungal infections can include rashes or a mild respiratory illness while others can become sepsis, meningitis, or pneumonia.

Mycotic (or fungal) infections pose an increased risk to the public for a number of reasons, which include:

  1. Opportunistic infections such as cryptococcosis and aspergillis are on the rise for those with weakened immune systems (the very old, the very young, cancer patients, those with immunodeficiencies).
  2. Nosocomial Infections (also known as “hospital acquired infections“) such as candidemia, are one of the leading causes for blood-borne infections in the US. Thanks to advancements and changes in healthcare practices, there are increased opportunities for new, drug-resistant fungi to emerge in hospital and hospital-like settings (like a nursing home).
  3. Community-Acquired Infections (like histoplasmosis, blastomycosis, and coccidioidomycosis) are caused by environmental fungi that live in the soil, on plants, or in compost heaps, and are common throughout the US. The changes in climate may be affecting these fungi.

Parasitic Infectious Diseases:

A parasite is an organism that lives on – or inside – a host organism and gets its food at the expense of the host. Parasitic infections are a huge problem in the tropics and subtropics, as well as more temperate locals. Of all parasitic diseases, malaria causes the most deaths worldwide, killing about 1 million people (mostly young children) in sub-Saharan Africa.

However, parasitic pathogens also affect the developing countries. In the US, trichomoniasis is the most common parasitic infection, followed by Giardia and Cryptosporidium infections.

The most common parasitic disease-causing organisms in humans are:

1) Protozoa – these microscopic, one-celled organisms can be parasitic OR free-living in nature and can multiply in humans, continuing their survival while allowing very serious infections to develop. Transmission from the human GI tract generally occurs through the fecal-oral route, while transmission from protozoan blood disorders is transmitted via arthropod vector (mosquito, fly, or other insect). The four types of protozoa that are infectious to humans are classified based upon their movements:

  • Cilliophoria – the cilliates, such as Balantium, are characterized by hairlike organelles called “cilia,” which are used for swimming, crawling, feeding, sensation, and attachment.
  • Mastigophora – the flagellates, such as Giardia, Leishmania, have one or more whip-like organelles that are used for movement.
  • Sarcodina – the amoeba, such as Entamoeba, use their pseudopods – lobe-like bulges from the cell membranes – that are used for locomotion and feeding.
  • Sporozoa – adult stage is immotile, such as Plasmodium, Cryptosporidium, which means that they lack any locomotor organs.

2) Helminths – these large, mulch-cellular organisms are usually seen with the naked eye in its adult stages. Similar to protozoa, helminths can be parasitic or free-living, but adult helminths cannot reproduce in humans. The three primary groups of helminths that can cause parasitic infections in humans are:

  1. Platyhelminthes (flatworms) – include both trematodes (the flukes) and cestodes (tapeworms!) and are simple, unsegmented, soft-bodied animals without body cavities or circulatory systems.
  2. Acanthocephalins (thorny-headed worms) – adult versions of these worms live in the gastrointestinal tracts and are considered to be intermediate between cestodes and neatodes.
  3. Nematodes (roundworms) – adult versions of these worms live in the GI tract, blood, lymphatic system, or subcutaneous tissues.

3) Ectoparasites – blood-sucking arthropods that burrow into the human skin and reside their for a long timeframe and include, ticks, fleas, mites, and lice. Not only can these ectoparasites cause diseases, they may also act as biological vectors in transmission of different pathogens that can lead to diseases that cause high mortality and morbidity.

Prion Infectious Diseases:

Prion Diseases (also known as transmissible spongiform encephalopathies or TSEs) are a family of rare, yet progressive, neurodegenerative disorders that affect humans and animals. Prion related infectious diseases are distinguished from other pathogenic diseases by the long incubation periods, characteristic spongiform changes with associated neuronal loss, and a failure to induce an inflammatory response.

Prions, transmissible, abnormally-folded, disease-causing agents, cause abnormal folding of specific normal cellular proteins called “prion proteins,” found most frequently in the brain, are suspected as the causative agents for TSEs. This folding of the prion proteins causes progressive brain damage and death.

The most well-known prion disease is Creutzfeldt-Jakob Disease Disease (CJD) or mad cow disease.

How Are Infectious Diseases Diagnosed?

If an infectious disease is suspected by a physician, your doctor will take a complete medical history and order some laboratory tests or imaging scans to determine the cause for the symptoms. The following tests may be utilized to ascertain the particular infectious agent:

Laboratory Tests:

  • Laboratory blood tests – a sample of blood may be taken by inserting a needle into the vain.
  • Urinalysis – urinating into a container, avoiding contamination to ascertain if there’s something growing in the urinary tract.
  • Throat swabs – samples of bacteria at the back of the throat may be taken using a sterile swab.
  • Lumbar puncture – also known as a spinal tap, this procedure obtains a sample of cerebrospinal fluid through a needle inserted between the bones of the spine.

Imaging Scans:

  • X-Ray – a small dose of radiation is aimed at the affected part of the body to reveal images of the structures within the body, such as pneumonia.
  • CT Scan – a CT scan digitally combines X-rays taken from a number of angles to produce cross-sectional images of the bones, organs, and soft tissues.
  • MRI Imaging – radio waves combined with a strong magnetic field produces detailed images of the internal structures.


a small sample of tissue is removed from an internal organ to be tested by the pathology laboratory to detect the presence of certain diseases.

How Are Infectious Diseases Treated?

Infectious diseases are treated in accordance to the type of disease causing the infection.

Bacterial Infections are treated with antibiotics and grouped into families of similar types of antibiotics. Certain classes of bacteria are more susceptible to certain types of antibiotics. Treatment is often targeted at the infectious disease present.

It’s important to note that the overuse of antibiotics has resulted in antibiotic resistant bacteria, which makes treatment of the infectious diseases more difficult.

Viral Infections are occasionally treated with antiviral medications. While there are some medications that can be used to treat viruses, but not all viruses have a corresponding medication. Examples of drugs that are targeted to viruses include:

  • AIDS
  • Herpes
  • Hepatitis B
  • Hepatitis C
  • Influenza

Fungal Infections can infect the lungs or the mucous membranes of the mouth and throat, often in those who are immunocompromised. Anti-fungal medications are the treatment of choice for those with fungal infections.

Parasitic Infections are diseases that are caused by parasites are often treated with anti-parasitic medications. It’s important to note that many varieties of parasites have developed a resistance to the anti-parasitic medications.

How Are Infectious Diseases Prevented?

One of the most important ways to prevent – or slow – the transmissions of infectious diseases is to understand and recognize the characteristics of the different diseases. These characteristics include the virulence, distance traveled by victims, and level of contagiousness.

Another way to prevent the transmission rates of infectious diseases is to recognize the way that diseases can jump between hubs – groups of infected individuals.

Know the modes of transmission. Infectious diseases can enter the body via:

  • Skin contact or injuries to the skin
  • Inhalation of airborne pathogens
  • Ingestion of contaminated water or food
  • Sexual contact
  • Tick or mosquito bites

These tips can reduce the risk of spreading infectious diseases to you and others around you:

  • Wash hands, especially after using the bathroom, preparing food, before eating.
  • Be sure to rest as much as possible.
  • Keep hydrated – drink water or sports drinks.
  • Practice proper food preparation – keep counters and surfaces in the kitchen clean while preparing foods.
  • Quickly refrigerate leftovers – no food should be left out of the refrigerator at room temperature for long periods of time.
  • Ensure vaccinations are up-to-date, including a yearly flu shot and making sure your tetanus vaccination is current.
  • Don’t leave the house – don’t go to work or school if you’re running a high fever, vomiting, have diarrhea. Keep your child home as well.
  • Always practice safe-sex. Use condoms and birth control even if you or your partner doesn’t engage in high risk sexual behavior.
  • Travel smart – don’t get on a plane if you’re sick. You’ll merely infect other passengers and your trip will probably not be much fun if your sick.
  • When traveling, ensure that you’re up-to-date on your vaccinations.
  • Don’t share your personal items with other people. This includes drinks, toothbrushes, silverware, razors, combs and other personal hygiene items.

Additional Infectious Disease Resources:

National Prion Disease – surveillance and research site supported by CDC to uncover more information about prion diseases.

Emedicine’s List of Infectious Disease Articles – a thorough A-Z list of infectious diseases with links to various articles.

World Health Organization – information, links, and articles related to various types of infectious diseases from around the world.

Infectious Disease Society of America – represents physicians, scientists and other health care professionals who specialize in infectious diseases. IDSA’s purpose is to improve the health of individuals, communities, and society by promoting excellence in patient care, education, research, public health, and prevention relating to infectious diseases.

Page last audited 8/2018