What Are Infectious Diseases

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Overview: what are infectious diseases? 

Infectious diseases are everywhere. Have you ever hiked through woodlands and been warned to check yourself or your dog for ticks? These small creatures are notorious for spreading the bacterial Lyme disease. Did you stay home when you had measles, or isolate during the COVID-19 pandemic? These are all infectious diseases, and can easily spread through various means such as airborne transmission, contaminated food and water, and even through body fluids and sexual contact. Due to their potential to cause widespread outbreaks, these diseases are classified as communicable. 

Infections from  microorganisms such as bacteria, fungi ,viruses, and parasites are associated with dysfunction of the infected individual (be that plant, animal, or human), and the manifestation of illness in many cases. Consequently, the prevention and control of communicable disease is paramount in maintaining public and environmental health. In this article we will broadly cover the topic of infectious diseases, providing you with a comprehensive understanding of their characteristics, transmission mechanisms, and impact on health and wellbeing.

Types of infectious diseases

There is a diverse list of infectious diseases caused by pathogenic organisms (those which cause disease to the host). The most common infections are caused by bacteria and viruses. Some examples are discussed below: 

Bacterial:  Caused by bacteria present in the environment, air, food, water, and bodily fluids. 

Pertussis: Known as whooping cough, pertussis is a highly contagious infection of the respiratory tract. It is caused by a bacterium called Bordetella pertussis, which only affects humans- primarily infants. It can last for several weeks. The bacterium enters the respiratory tract through the air via the nasal membrane. It travels down the respiratory tract where it sticks to cilia (hair like structures that can trap bacterium) on the tissue membrane. These bacteria start to rapidly multiply or proliferate at the trachea and then descend to stimulate mucus secretion. This leads to paroxysmal bouts of irregular or violent coughing,causing  the infected individual to draw in large amounts of oxygen through a “whoop”. It is still present in the UK, though its presence has declined due to vaccination programs.²,³

Typhoid: Typhoid or enteric fever is a common foodborne illness that spreads through contaminated food and water. The bacteria that cause this are Salmonella typhi and Salmonella paratyphi. In the UK, typhoid is a rare disease but it is still prevalent in Africa, Asia, and South America in areas of underdeveloped sanitation infrastructure. The disease is controlled using two vaccinations. A typhoid infection is often coupled with  gradually increasing fever, persistent headache, and constipation. Complications include diarrhoea and rashes and, if left untreated can lead to death. One of the best ways to prevent typhoid and improve public health is to improve food safety practices by maintaining good hygiene and preventing water pollution.⁵


Viral: Viruses are non-living particles that are even smaller than bacteria and require a host to replicate. Disease is caused when viral replication disrupts the host’s regular cellular and organ function. Some well-known viral infections include: 

  1. Measles: Measles is a common childhood infection caused by species of the Morbillivirus genus. It is a highly infectious  acute respiratory illness that results in a characteristic blotchy red rash on the skin, a high fever, sore throat, and inflamed eyes. Symptoms present  after fourteen days of exposure to the virus, though the disease is transmissible before symptoms begin to show. Occasionally, severe symptoms such as encephalitis (swelling of the brain) and pneumonia are present and life-threatening⁶
  2. Mumps: Mumps, or parotitis, is a sporadic airborne disease caused by paramyxovirus of the Rubulavirus family. Affecting the parotid salivary glands, this infection causes painful inflammation and swelling of the jaw. Mumps mainly affects young  children and is rarely seen in adults. Symptoms include a high fever, inflamed parotid glands, and joint pain. If the virus extends to the brain, meningitis is a life-threatening possibility⁷
  3. Rubella: Rubella is an airborne respiratory disease caused by the Rubella virus.  It is also known as German measles, or three-day measles, and procures intensely red rashes that spread from the face to the trunk and limbs, lasting for three days. Rubella can also be congenital (If a pregnant woman is affected, the disease can be present in the baby at birth). Complications include encephalitis  and hearing loss¹³

Measles, mumps, and rubella are being eradicated worldwide through administration of both doses of the MMR vaccine, introduced in 1988. Vaccination efforts allowed for the eradication of measles in the UK in 2017, and continued uptake of the vaccine is required to ensure this highly infectious disease stays eliminated. 

  1. SARS: Severe Acute Respiratory Syndrome caused by SARS-CoV-1 (Severe Acute Respiratory Syndrome Coronavirus 1) is a highly contagious airborne respiratory illness that made its first appearance in an outbreak in 2003, with the epicenter of infection being China. SARS is spread through cough droplets and saliva of the infected person and through touching surfaces. After the 2002-2004 outbreak, the disease has not been detected anywhere in the world. It produces symptoms similar to the flu such as sore throat, high fever of 40°C, fatigue, cough, etc¹⁴
  1. HIV/AIDS: This highly contagious  Sexually Transmitted Disease (STD), caused by the Human Immunodeficiency Virus (HIV) is transmitted through unprotected sex and spreads through body fluids such as blood, semen, and vaginal fluids. It is a disease that has garnered a high level of social neglect and discrimination among those infected. HIV causes the disease Acquired Immunodeficiency Syndrome (AIDS), which is a disease with long latency, showing symptoms up to decades after infection. Those infected have a gradually ineffective immune system, resulting in weight loss, muscle weakness, swelling of the lymph nodes, and often leading to mortality via infection with a secondary pathogen

Risk factors for infectious diseases

One of the primary risk factors for catching an  infectious or communicable disease is lack of access to sanitation infrastructure, such as restrooms and hand/food washing stations. In cases such as typhoid and poliomyelitis, poor sanitation is a contributing factor since transfer is associated with fecal matter incorporation into food or water nearby. Other forms of infections such as Norovirus and Salmonella are due to cross-contamination, unsanitary food conditions, and lack of appropriate food storage (such as in fridges and freezers). 

Another important risk factor for infection is a lack of social disease awareness leading to lack of preventative measures and low uptake in treatment such as antibiotics and vaccines. Diseases like measles, mumps, and rubella, which are prevalent in young children, are more prone to spread quickly due to the high density in schools,  and lack of hygiene knowledge; going  undetected until visible symptoms are seen. 

Unprotected sex with multiple sexual partners is a very high risk factor for preventable sexual diseases, many of which are resistant to antibiotics and are becoming increasingly harmful for human health. People with harmful lifestyle habits such as smoking and excessive alcohol consumption are at high risk for respiratory illnesses and Hepatitis A, respectively.

In animal-associated diseases (zoonotic diseases)  such as rabies, a neglected tropical disease, contact with the saliva or mucus of wild animals via bites and scratches can cause rapid infection of the nervous system. Human imposition on natural ecosystems (such as deforestation or hunting), alongside unnecessary interaction with wild animals, are risk factors for zoonotic disease transmission.  

Even a change in location without adequate protection, such as traveling to a foreign country without receiving the appropriate vaccinations, can result in disease. Some diseases are more prevalent in some parts of the world, so research is required before travel to ensure the safety of yourself and others. Crowded areas, such as airports and public transportation, also pose a risk for infection if an affected individual is in close proximity with others. 

Signs and symptoms of infectious diseases

Symptoms (experienced by the infected individual) and signs (noticed by those around the individual) of infection vary with the disease that is present. Usually, the first noticeable symptom of any infectious disease is fever; this is an indication that the body is raising the internal temperature to fight and kill and infection b. When a fever starts to drain, it is accompanied by chills and sweat. 

Diseases spread by droplets of mucus or saliva may also display changes in the skin; with measles, red rashes are an indicator. Chickenpox is also confirmed by the appearance of itchy white rashes that look like blisters. Infectious rashes can be either fluid-filled or not, which aids diagnosis.⁸,⁹

In cases of typhoid and cholera, diarrhoea and loss of appetite are seen along with dehydration. 

In sexually transmitted diseases, such as syphilis and chlamydia, common symptoms include pain during urination and abnormal discharge from the vagina or penis. This may be accompanied by a change in skin composition;in cases of genital herpes, some fluid-filled bumps or ulcers are present near the groin. Since many signs of infectious disease  are visible, a disease may be detected visually (though infection may still occur before signs appear) alongside general malaise. Disease symptoms may also vary as skin colour does, so consulting a health professional is always advised for a diagnosis. 

Management and treatment for infectious diseases

Patients with communicable diseases must practice good hygiene, such as hand washing before eating and using the bathroom, and using tissues to catch infectious droplets. In the case of high disease severity, they may be required to quarantine  at home for the well-being of others. Cases of vector-borne diseases such as those transmitted by mosquitoes (malaria, dengue, yellow fever, etc.), sandflies (leishmaniasis), tsetse flies (sleeping sickness), etc., can be controlled by using insect repellents and by reducing vector proliferation. In the case of malaria, reducing the presence of stagnant water (the breeding ground for mosquito larvae) can aid in disease control. 

Popular modes of infectious disease treatment include:

Antibiotics - are chemicals obtained from living sources such as fungi that kill pathogenic bacterial populations exclusively in the case of bacterial infections. Eventually, the death of infectious bacteria decreases disease symptoms in the treatment process. . Antibiotics can be administered orally (tablets, pills), intravenously, or by external application (creams).¹⁴

Vaccines - are used as both treatment and prevention for infectious disease. Vaccines contain dead and weakened versions of the disease-causing organism/toxin. When a vaccine is administered, the body’s immune system gains an introduction to the pathogen and begins producing antibodies specific to the disease. If the real disease is encountered, the affected person is more able to fight the infection before it becomes transmissible, reducing symptom severity though immunological memory cells. ⁽¹⁴⁾ For example, Diphtheria, tetanus, and whooping cough can be prevented by the DTaP vaccine.

Routine vaccination plans have been extremely effective in treating, even eliminating, diseases worldwide. For example, Poliomyelitis (Polio virus), was declared extinct in the UK in 2003 through vaccination efforts, and worldwide eradication is a public health target. 

While the Human Immunodeficiency Virus (HIV) has impacted populations for several decades, a lack of awareness of the disease and social isolation still linger worldwide. HIV/AIDS has been the main target for education and public health improvement strategies in many countries. 

Antibacterial resistance? 


Certain tests are required for diagnosis, for which the patient’s blood, mucus, saliva, urine, or genital fluids are usually drawn in order to identify the disease-causing organism. Simple bacterial staining such as Gram staining, alongside biochemical tests, can be carried out for the identification of the type of bacterium responsible. Unique tests are also available; for typhoid, a WIDAL test is done. The test checks for the presence of antibodies specific to Salmonella typhi and Salmonella paratyphi. In retroviral diseases like HIV/AIDS, an Enzyme-Linked Immunosorbent Assay (ELISA) is carried out to detect antibodies in the patient’s blood. In the case of a HIV-weakened immune system, antigen tests can test directly for the presence of specific HIV markers.¹¹


Infectious diseases can lead to health complications and susceptibility to secondary infection. In the case of tetanus, an infection caused by the bacterium Clostridium tetani, lockjaw can develop if the disease is left undetected. Some untreated bacterial infections such as pneumonia can lead to sepsis as bacterial infection spreads from the lungs to the bloodstream. In the case of bacterial meningitis (inflammation of the protective brain meninges), serious complications include stroke, paralysis, brain damage and eventual mortality. 


How can I prevent infectious diseases?

Infectious diseases can be prevented by practicing good hygiene such as hand washing before eating and using the bathroom. Keeping away from an infection through isolation of the infected individual is recommended for the health of others. Always let your peers know if you suffer from any symptoms- it is essential that you protect yourself and others from disease. 

How common are infectious diseases?

Humans, alongside often living in densely populated areas, are a natural reservoir for a huge range of microorganisms. Hence, both direct and indirect contact can lead to the spreading of diseases. Infectious diseases are very common amongst human and animal populations globally though they are not always life-threatening. 

How do infectious diseases spread?

Infectious disease can spread through aerosols like coughing or sneezing from an infected individual through the air. It can also spread through contaminated water and food, and by touching surfaces that an infected individual had previously touched. Unprotected sex and having multiple sexual partners can lead to the contraction of Sexually Transmitted Infections (STIs). Blood infections can occur through open wounds and the sharing of needles.

When should I see a doctor?

A doctor’s appointment is recommended in the case of persistent fever and malaise, muscle weakness, abnormal skin changes (rashes, warts), or if you have been in contact with a known infected individual. 


Infectious diseases are maladies often caused by microorganisms (commonly bacteria) and viruses as they enter the body, that are communicable as able to be passed from one person to another. After contracting an infection, the individual may see symptoms that vary with the disease entry and modes of transmission and replication. These often include fever, loss of appetite, and cough, though additional changes in skin physiology are not uncommon . Due to rapid spreading, infectious diseases have the ability to cause outbreaks worldwide. As a result,  the maintenance of public and environmental health are essential for the control of infectious outbreaks. Prevention and treatment for infectious diseases are becoming more widely researched and applied- in some cases, diseases can even be eradicated. Infection control, alongside the improvement of infrastructure, healthcare, and societal awareness of important diseases, pave the way for an increasingly healthy population worldwide. 


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  3. Pertussis (Whooping cough) | concise medical knowledge [Internet]. 2020 [cited 2023 Jul 1]. Available from: https://www.lecturio.com/concepts/pertussis-whooping-cough/
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  8. CDCBreastCancer. Centers for Disease Control and Prevention. 2021 [cited 2023 Jul 1]. Know the signs and symptoms of infection. Available from: https://www.cdc.gov/cancer/preventinfections/symptoms.htm
  9. nhs.uk [Internet]. 2017 [cited 2023 Jul 1]. Sexually transmitted infections (Stis). Available from: https://www.nhs.uk/conditions/sexually-transmitted-infections-stis/
  10. Drexler M, Medicine (US) I of. Prevention and treatment [Internet]. National Academies Press (US); 2010 [cited 2023 Jul 1]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK209704/
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  12. cycles T text provides general information S assumes no liability for the information given being complete or correct D to varying update, Text SCDM up to DDTR in the. Statista. [cited 2023 Jul 1]. Topic: communicable diseases in the uk. Available from: https://www.statista.com/topics/5989/communicable-diseases-in-the-uk/
  13. Rubella [Internet]. [cited 2023 Jul 2]. Available from: https://www.who.int/news-room/fact-sheets/detail/rubella
  14. Severe acute respiratory syndrome(Sars) [Internet]. 2020 [cited 2023 Jul 2]. Available from: https://www.hopkinsmedicine.org/health/conditions-and-diseases/severe-acute-respiratory-syndrome-sars

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This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Dharshana Guru Raghavendran

MSc. Infection, Immunity and Infectious Disease - University of Leeds, United Kingdom

Dharshana is a researcher in the field of immunology. She’s especially passionate about studying auto-immune conditions, hypersensitivity, and gastrointestinal disorders.

Dharshana is also an experienced scientific communicator and has helmed many research projects as well as management roles.

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