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Infectious Diseases


The entry of any living organism into the body of a person is called infection. Diseases that are caused by the entry of the living organism into the body of a person are called infectious diseases. These are the diseases caused by living organisms like bacteria, fungi, viruses, or parasites. The organisms responsible for infectious diseases are called pathogens.

Pathogens causing infectious diseases have certain pathogenic factors that can cause disease processes once they enter the body. The virulence of an organism depends on these disease-causing pathogenic factors.

Infectious agents can also spread from one organism to another. Thus, infectious diseases can be transmitted from one person to another. Different pathogens have different modes of transmission. Certain infectious diseases can be prevented by the use of vaccines.

In this article, we will talk about types of infectious diseases, signs and symptoms, investigations, and their management.


Infectious diseases are divided into different categories on the basis of the type of pathogen. They are broadly divided into the following four categories.

  • Bacterial diseases caused by bacteria
  • Viral diseases caused by viruses
  • Fungal diseases caused by fungi
  • Parasitic disease caused by parasites

We will now talk about some of the common infectious diseases caused by these pathogens.

Bacterial Infectious Diseases

Millions of bacteria are present in the human body. They live on the skin, in the oral cavity, in the gut and in other sites of the body. This is known as the normal flora of our body. Humans and bacteria live in a symbiotic relationship with one another.

Our body has a number of protective mechanisms to protect us from the harmful effects of bacteria. Bacterial infections result when there is a breach in any of the protective mechanisms causing a disturbance in the normal flora of our body.

A breach in the skin can cause the introduction of bacteria into the bloodstream that can cause disease. Long term use of antibiotics may disturb the normal flora causing the death of some bacteria while others may become more potent. A weak immune system will allow foreign bacteria to pass unnoticed that can cause disease.

The most important infectious disease caused by bacteria is tuberculosis, meningitis, syphilis, endocarditis. A brief detail of these diseases is discussed below.

Read more about the Immune System


It is a systemic infectious disease caused by Mycobacterium tuberculosis bacteria. This organism causes more deaths worldwide as compared to any other microorganism. Almost one-third of the population of the world is infected with this organism.

Signs and Symptoms

Multiple organs can be involved in this disease. Fever, fatigue, weight loss and night sweats are common general symptoms. Cough and hemoptysis are seen in pulmonary tuberculosis. Pleural effusion may also be seen in such patients. Chest X-ray shows cavities in the lungs.

Tuberculosis is an example of an infectious disease caused by bacteria


Mycobacterium tuberculosis is transmitted from person to person via respiratory droplets through coughing. Humans are the natural reservoirs. The portal of entry is the respiratory tract. Lungs are the first site in the body to be infected with the virus.


The disease is confirmed by acid-fast staining of the sputum culture. If the culture is positive, the person is said to have the disease.

The disease can be treated via multidrug therapy. Treatment involves using three drugs; isoniazid, rifampin, and pyrazinamide. These drugs can also be used for prophylaxis in persons with positive contact history.


It is an infectious disease that occurs due to bacterial infection of the cerebrospinal fluid. It is the inflammation of the meninges. It is caused by Neisseria meningitides, a gram-negative coccus.

Signs and Symptoms

The disease is characterized by headache, nausea, vomiting and malaise. Fever with chills and neck stiffness are the presenting symptoms in children. In the advanced stage, it can cause irritability, sensitivity to light, lethargy, and purple bruises on the skin.


The bacteria are transmitted from person to person via respiratory droplets. It easily spreads to people living in highly congested areas. There is no animal reservoir for this pathogen.


Penicillin G is the treatment of choice for patients infected with Neisseria meningitides. A third-generation cephalosporin such as ceftriaxone can also be used for treatment purposes.


It is a bacterial infection caused by Treponema pallidum. It is a sexually transmitted disease that affects the genital system of men and women. It is more commonly seen in sexually active people who do not use any kind of protection.

Signs and Symptoms

Based on the clinical findings, syphilis is divided into three types; primary, secondary, and tertiary.

Primary syphilis is characterized by the formation of non-tender ulcers at the site of entry. These ulcers are seen on the genital skin and are formed due to the multiplication of bacteria at the entry point.

Bacteria soon spread via the bloodstream and produce multiple systemic symptoms. One to three months after the first infection, lesions of secondary syphilis can be seen. Secondary syphilis is characterized by a maculopapular rash on the surface of palms and soles. Moist papules can also be seen on genital surfaces, skin, and mucous membranes.

If left untreated, the disease can develop into tertiary syphilis. It is characterized by granulomas of skin and bone. In this stage, the disease also involves the central nervous system and the cardiovascular system. Balloon like dilation of ascending aorta is also seen at this stage of the disease.


The active lesions formed on the skin and mucous membranes contain Treponema pallidum. They can be transmitted from one person to the other via intimate contact. Unprotected sexual intercourse is the most common cause of syphilis. Sometimes, the responsible bacteria are also present in the blood. Thus, the disease may also spread via blood transfusions.


Penicillin G is the drug of choice for the treatment of syphilis. However, in some patients, the use of penicillin can cause fever, chills, and myalgias in the patients. This is known as the Jarisch-Herxheimer reaction.

The disease can be prevented by using condoms and the administration of prophylactic antibiotics after suspected exposure.

Infective Endocarditis

It is an infection of the endocardium. The endocardium is the innermost layer of the heart that lines the inner chambers as well as heart valves. Infective endocarditis is the inflammation of the endocardium that lines the heart valves. In most cases, it is due to some bacterial infection.


In most cases, infective endocarditis is caused by infection of the endocardium by these four bacteria.

  • Streptococcus viridians
  • Streptococcus bovis
  • Staphylococcus aureus
  • Staphylococcus epidermidis

Signs and Symptoms

It is characterized by fever due to bacteremia. Murmurs are also heard due to vegetations on heart valves. Janeway lesions are erythematous non-tender lesions on the palm and sole seen in endocarditis. Osler lesions are also seen that are tender lesions on fingers and toes. Splinter haemorrhages in nail beds are also seen. Anaemia of chronic disease is also seen due to chronic inflammation of the endocardium.


Antibiotics are prescribed for the treatment of bacterial infections. Surgery is planned if there is heart failure, valvular obstructions, or repeated emboli.

Viral Infectious Disease

Viruses are facultative parasites. They cannot survive outside the bodies of their hosts. Unlike bacteria, viruses do not live in a symbiotic relationship within the human body. They are always introduced in the body from an external source to cause disease process.

Important viral diseases include hepatitis, influenza, AIDS, etc.

Viral Hepatitis

It is an inflammation of the liver caused by the hepatitis virus. It can occur due to one of the following viruses:

  • Hepatitis A virus
  • Hepatitis B virus
  • Hepatitis C virus
  • Hepatitis D virus
  • Hepatitis E virus

A and E cause acute disease, while B and C viruses cause chronic hepatitis. Hepatitis D causes subacute hepatitis.

Signs and Symptoms

Acute disease is characterized by fever, malaise, lethargy, abdominal pain, and nausea. In later stages, marked jaundice is seen in patients.

Chronic disease is characterized by hepatosplenomegaly and ascites. In advanced stages, fibrosis of the liver causes liver cirrhosis. Liver functions are markedly decreased with oedema, clotting disorders and pleural effusion.


As mentioned earlier, hepatitis is caused by five different types of viruses. Each of these viruses has its own mode of transmission.

Hepatitis A and E spread via the faecal-oral route. It spreads by drinking impure water that has been contaminated with human stools.

Hepatitis B, C and D spread via the exchange of body fluids such as blood and semen. Unprotected sexual intercourse, blood transfusions and can cause the spread of these species.


Acute hepatitis can resolve without any medication. Antiviral therapy is used to treat chronic forms of hepatitis.


Influenza is a common viral infection throughout the world. It affects around 5-10% of adults and 20-30% of children every year. It is an acute viral infection of the lungs and airways.

Sign and Symptoms

In the case of seasonal influenza, the patient presents with fever, dry cough, and sore throat. There is associated headache, myalgia, pain in the eyes, and photophobia. In most cases, it is a self-limiting disease. However, in some patients, it can complicate pneumonia and chronic lung disease.


Influenza virus rapidly spreads from person to person via air droplets. Close contact with the already infected person.


Uncomplicated influenza requires symptomatic treatment with paracetamol. Complicated influenza causes infection of the lower respiratory tract. It requires treatment with specific antiviral drugs.


Acquires immunodeficiency syndrome (AIDS) is a viral infection caused by the Human immunodeficiency virus (HIV). It is a syndrome in which the immune system of the body is heavily compromised. A compromised immune system leads to an increased risk of opportunistic infections and malignancy.

Signs and Symptoms

The patient infected with HIV is symptomatic in 80% of the cases. The symptoms appear 2-4 weeks after primary HIV infection. The patient presents with fever, rash, myalgias, pharyngitis, mucosal ulcerations, headache, lymphadenopathy, and meningitis.

The patient may present with other complications of the immune system.


HIV is transmitted by the transfer of body fluids such as blood and semen. Its primary transmission occurs by sexual contact and transfer of infected blood. It can also be transmitted from mother to neonate via transplacental transmission or after birth via breast milk.


Multiple antiviral drugs are used for the treatment of HIV/AIDS. Recall that HIV is a retrovirus having reverse transcriptase activity. Reverse transcriptase inhibitor enzymes are used for the management of the disease. No drug regimen results in a complete cure of the disease. However, chemotherapy has been successful in restoring immune functions and decreasing the viral load.

Herpes Simplex

It is a viral disease in which the epithelial cells of the skin are infected by the herpes simplex virus (HSV). There are two strains of the herpes virus: HSV1 and HSV2. HSV1 infects two-thirds of the world population. The virus multiplies within the epithelial cells of the skin and mucosal surfaces and gives rise to vesicles or ulcers.

Signs and Symptoms

The patient presents with tingling of the skin and vesicles formation. The vesicles later convert into ulcers. The lesions may be seen on lips, genital surfaces, in the cornea of the eye, etc. The patient may present with systemic signs of the disease that include fever and sore throat.


HSV1 is transmitted in saliva, while HSV2 is transmitted via unprotected sexual contact. This provides a reason why HSV1 lesions occur on lips while HSV2 lesions occur in the genital area.


Antiviral therapy is used to treat the disease. Acyclovir is the drug of choice for the treatment of HSV1 infection. It provides a cure to the local as well as systemic manifestations of the disease.

Fungal Infectious Disease

Fungi are eukaryotic organisms that exist in the environment in two forms; yeasts and moulds. Yeasts are single cells, while moulds consist of long filaments made up of multiple fungal cells. They reproduce by spore formation. Spores of different fungi are abundantly present in the environment. Fungal infections occur via the entry of these spores into the human body.

In most individuals, spores are readily detected and are made harmless by an immediate immune response. Fungal infections are most commonly seen in individuals having a defective or non-functional immune system.

Some important disease caused by fungal infections is as follows.


It is an infectious disease caused by Histoplasma capsulatum. This fungus is present in many parts of the world. It grows in soil that is particularly contaminated with bird droppings.

Signs and Symptoms

In most cases, the person infected with H. capsulatum remains asymptomatic. Some patients may present with severe pneumonia as the fungus causes cavitary lesions in the lungs. In immune-compromised patients, disseminated histoplasmosis can develop having multiple systemic signs. Pancytopenia and ulcerated lesions on the tongue can be seen in such patients.


The infection takes place by the inhalation of fungal spores found in the environment. The spores are engulfed by macrophages, where they grow to form yeasts. In tissues, fungus exits in the form of yeast within the macrophages. Here, the yeast survives by inactivating the degenerative enzymes of phagolysosomes. The disease cannot spread from one person to another.


No particular treatment is necessary in the case of asymptomatic patients or those with mild infections. However, patients that present with severe lung lesions are treated with itraconazole. In the case of disseminated disease, itraconazole or amphotericin B is the drug of choice. It is given via the parenteral route.


It is a fungal infection caused by Candida albicans. The fungus can cause a number of symptoms in the infected person. Candida is a part of the normal flora of the upper respiratory tract, gastrointestinal tract, and female genital tract. Any break in the skin or mucous membrane results in the disease.

Signs and Symptoms

The patient infected with candida can present with oral thrush, vaginal thrush, vaginitis, or esophagitis. Infection in children can cause diaper rash.


As mentioned earlier, Candida is a part of normal flora. Any defect in the normal defence mechanisms can give rise to candidiasis. Skin invasion of the fungus occurs in warm and moist areas. Fingers and nails are infected when they are immersed in water for prolonged periods. Diaper rash occurs in infants when the wet diapers are not changed immediately. The risk of infection increases multiple times in patients with a compromised immune system. IV drug abuse and the use of catheters also predisposes to candidiasis.

Systemic disseminated disease occurs in patients that have HIV or are on immunosuppressant drugs.


The drug of choice for treatment is fluconazole. It is applied topically in case of local infections. Other antifungal drugs like voriconazole and itraconazole are also effective.


It is a fungal infection caused by Aspergillus fumigatus. This fungus can cause infections of the skin, eyes, ears, and lungs. The patient presents with multiple systemic signs and symptoms.

Aspergillus exists in the form of moulds that are widely distributed in nature. Infection occurs when they enter the human body.


Aspergillus infection occurs by the inhalation of airborne conidia. The conidia arise from the sporangium of fungus. Aspergillus can colonize the skin, burns, cornea, external ear, or paranasal sinuses. Later it can invade these surfaces and cause the disease.

Signs and Symptoms

The patient may present with itching in the eyes, pain in the ear, and skin lesions. In patients with a compromised immune system, the fungus can invade the lungs, causing hemoptysis. It results in the formation of cavities within the lungs. The fungus produces granuloma within these cavities called aspergilloma. These granulomas are called fungus balls. They appear as radiopaque opacities on chest Xray that change their position as the patient moves to one side or the other.


Voriconazole is the drug of choice for such patients. Fungus balls growing in the lungs can be surgically removed.


  • Infectious diseases are caused by disease-causing microorganisms that enter the body and escape the immune defence systems. They can be caused by viruses, bacteria, fungi, or parasites.
  • Bacterial infections occur due to a defect in the normal defence mechanisms that result in disturbance of the normal flora.
  • Tuberculosis is a bacterial infection that mainly affects the lungs and causes respiratory symptoms. It can also affect other organs. It spreads via aerosol droplets from the carrier or the patient.
  • Meningitis is an inflammation of the meninges that produces CNS symptoms. It occurs due to bacterial infection of the CSF. Neisseria meningitides is an important causative organism.
  • Syphilis is a sexually transmitted disease caused by infection with Treponema pallidum. The signs and symptoms vary depending upon the stage of the disease. It is transmitted via unprotected sexual intercourse.
  • Infective endocarditis is the bacterial infection of the endocardium of the heart. It results in CVS as well as systemic symptoms. Antibacterial therapy is used to treat the disease.
  • Hepatitis is a viral infectious disease that affects the liver. Five different strains of viruses can cause hepatitis. Each strain has its own mode of transmission. Acute hepatitis has easy recovery, while chronic hepatitis causes more serious disease.
  • AIDS is a syndrome caused by the HIV virus. This virus infects the helper T-lymphocytes, and the entire immune system becomes compromised. The patient becomes highly susceptible to a number of opportunistic infections and malignancies.
  • Histoplasmosis is a fungal infectious disease. In severe cases, it can cause pneumonia with cavitary lesions in the lungs.
  • Candidiasis is another fungal infection that has multiple manifestations. The signs and symptoms depend on the part of the body that got infected with fungus.
  • Aspergillosis is characterized by fungal infection in the eyes, ears, and lungs. Respiratory infection is more dangerous. It results in the formation of granulomas within the lungs called fungal balls. These granulomas can be removed surgically for complete recovery.

Frequently Asked Questions

What are the 5 most common infectious diseases?

The prevalence of infectious diseases depends on certain demographic factors. However, the five most commonly seen infectious diseases include Hepatitis B, Tuberculosis, COVID-19, Dengue, and Malaria.

What are 4 types of infections?

Based on the type of pathogen, infections are of four types. Bacterial infections are caused by bacteria. Fungal infections are caused by different fungi. Parasites are responsible for parasitic infections. Viral infections are caused by certain viruses.

What are the signs of infection?

The signs of infection majorly depend on which organ system is infected. For example, in lower respiratory tract infections, the patient will experience cough with sputum, shortness of breath, etc. However, general signs of infection include fever, rigors and chills, fatigue, sore throat, etc. 

How to treat a bacterial infection?

Bacterial infections can be treated by using antibiotics. These are the drugs that either inhibit bacterial growth or destroy them by using different mechanisms. Most commonly used antibiotics include penicillin, cephalosporins, macrolides, fluoroquinolones, etc.


  1. Review of Microbiology and Immunology by Warren Levinson
  2. Oxford Handbook of Clinical Medicine