Contagious diseases

Definition

Contagious diseases are those diseases caused by pathogenic (disease-causing) agents, such as bacteria, viruses, and fungi, that infect the body. Contagious diseases, also called communicable diseases, are spread from person to person through direct contact or through contact with body fluids.

Distinguishing Features

Microorganisms are invisible and abundant residents of every habitat on Earth. Many have adapted to live inside the human body, and the vast majority reside there harmlessly or even with great benefit to the host. A small fraction of these microbes are pathogenic and can lead to infectious diseases in humans. An even smaller number of pathogenic microbes can be transferred directly from one person to another, causing diseases that are termed “contagious.”

Not all infectious diseases are contagious. One example of an infectious but not contagious disease is malaria, caused by the protozoan Plasmodium falciparum, which is transferred from one person to another by the bite of a mosquito. Direct contact with an infected person or with that person’s body fluids will not spread this disease. In contrast, a notoriously contagious infectious disease is influenza, which is caused by an orthomyxovirus that is easily spread from one person to another.

Biology of Pathogenic Organisms

There is great diversity among the types of pathogenic microorganisms. Contagious diseases are caused by viruses, bacteria, fungi, protozoa, and helminths (parasitic worms). The specific biology of each organism determines what part of the body it will infect, what symptoms it will cause, how it will spread from one person to another, and how it can be treated.

Viruses. Viruses are the smallest and simplest of all the pathogens that cause contagious diseases. They do not have cells and are not considered to be living organisms. Viruses are effectively designed containers, built to transport the genetic material they carry inside. They contain all the genetic information needed to re-create, so they have no materials or machinery for reproduction. After entering the body of a host animal, a virus reproduces by infecting a cell and taking over its reproductive machinery. New viral particles are assembled inside the cell and then released to the outside, where they can infect more cells.

Viruses cause a wide range of contagious diseases, many of which are serious or life-threatening. Treatment for viral infections is extremely limited or entirely unavailable. Because viruses exist inside human cells, it is difficult to destroy them without also killing the cells. Drugs that could destroy a virus would be extremely toxic or fatal to the infected person. Antibiotics work only on bacteria, making the human immune system the most critical factor in recovery from viral infections.

Bacteria. Bacteria are living single-celled organisms that are much more complex than viruses and about one hundred times larger. Bacteria are extremely significant to medicine, as they are responsible for large numbers of serious human illnesses. Bacteria are classified as prokaryotes because their cells have a simpler structure than those of plants and animals, which are eukaryotes. Despite their relatively simple structure, bacteria possess all the machinery necessary to grow and reproduce on their own. They are the smallest creatures on Earth that have this capacity. This feature sets them apart from viruses, which depend on host cells for reproduction. The small size and relatively simple structure of bacteria allows them to grow and reproduce rapidly.

A variety of antibiotics are available for the treatment of many bacterial diseases. These drugs target features of bacterial cells that are not present in eukaryotic cells. In this way, the drugs can kill the bacteria without damaging human cells. However, many bacteria have developed resistance to antibiotics. This worldwide problem limits the treatment options for a growing number of bacterial infections.

Fungi. Fungi are common eukaryotic microorganisms that only rarely cause disease in humans. Fungi have complex cells and a structure resembling those of plants. Unlike plants, fungi lack chlorophyll, so they cannot make their own sugars; they must live on nutrients found in their environment. Most take up residence on decaying plant matter and in soil. Fungi grow in two forms: molds and yeasts. Molds are quite common in nature. They are made of long filaments that branch and intertwine, creating the familiar mats that are often seen growing on bread and cheese. Yeasts are unicellular fungi that commonly live on fruits and flowers, thriving on the sugars provided there. Some are normal inhabitants of the human body.

Serious contagious diseases caused by fungi are quite rare in healthy persons. People who are seriously ill or have weak immune systems are more susceptible to fungal infections. Antifungal drugs are available and are effective against many fungal diseases.

Parasites. Parasites are eukaryotes that live on other living organisms for nutrition, without providing benefits to their hosts. Parasitic diseases are among the major causes of human suffering and death in the world. Contagious human parasites include protozoa and helminths. Protozoa are microscopic, unicellular eukaryotes, and as such are more complex than bacteria and have more in common with human cells. They are ten times larger than bacteria, and most can move or “swim.” These organisms feed by taking in fluid from their surrounding environment, and they reproduce inside the body of the host. Helminths are multicellular macroscopic worms that find nutrients in body fluids and intestinal contents. Unlike protozoan parasites, most helminths must leave the host to lay eggs, which are the infective forms of the organism.

Drugs are available for the treatment of parasitic infections, but few of them are ideal. Similarities between human and parasite cells make it difficult to design drugs that can kill parasites without also being toxic to humans. Some drugs require long-term administration, which is not practical in many developing nations.

Routes of Transmission

Pathogenic organisms vary in the way they spread from one host to another, a feature known as the route of transmission. For each pathogen, the route of transmission will determine where it enters and infects the body, how it spreads through a population, and how spread of the disease can be controlled. The routes of transmission for the agents of contagious diseases include airborne transmission, fecal-oral transmission, and direct transmission.

Airborne transmission. Airborne transmission occurs through the inhalation of infectious agents in aerosols that are released from an infected person. Aerosolized droplets are expelled by sneezing or coughing. The smallest of these droplets can remain suspended in air for a surprisingly long time (twenty minutes or longer). Inhaling the aerosol will introduce the pathogen into the respiratory tract. Different pathogens will infect different regions of the airways. Larger particles tend to settle from the air onto tissues sooner than do smaller particles, so the larger particles cause primarily upper respiratory infections. Smaller particles can infect the lower respiratory tract. Airborne pathogens can also be spread indirectly by contact with respiratory secretions that are on hands or inanimate objects. These organisms are then transferred to the airway through touching the nose, eyes, or mouth.

Organisms with airborne routes of transmission cause respiratory infections. The most common contagious infections worldwide are respiratory, because transmission through aerosols and contaminated objects occurs quite easily in normal daily activities. This mode of transmission is also known as casual contact. The majority of respiratory infections are caused by viruses. These include the common cold (rhinoviruses), influenza (orthomyxovirus), measles (paramyxovirus), and viral pneumonia (multiple virus types). Bacterial respiratory infections include tuberculosis (Mycobacterium tuberculosis), pneumonia (multiple species), strep throat (Streptococcus pneumoniae), and whooping cough (Bordetella pertussis). Some fungal diseases are respiratory. Most are caused by fungi that a healthy person’s body can fight without consequence. The more common fungal respiratory infections include valley fever (Coccidioides immitis), histoplasmosis (Histoplasma capsulatum), and cryptococcosis (Cryptococcus neoformans).

Fecal-oral transmission. Fecal-oral transmission is a common route by which many bacterial, viral, and parasitic diseases are spread. Organisms that are spread in this manner grow in the digestive tract, are present in feces, and usually cause diarrhea or vomiting. Infection occurs either by direct contact or through consumption of food or water that has been contaminated with human feces. Food can become contaminated by a food handler who is ill, particularly if the handler’s personal hygiene technique is inadequate. Raw shellfish, fruits, and vegetables that are washed in contaminated water can also spread disease. Waterborne pathogens are common in developing countries where sewage and drinking water are not treated. Natural disasters, such as earthquakes and floods, can breach water-treatment systems and cause outbreaks of waterborne illness.

Diarrheal diseases, which are the third leading cause of death in the world, are most often spread by fecal-oral transmission. Two of the most important are typhoid fever (Salmonella typhi) and cholera (Vibrio cholerae). These life-threatening bacterial illnesses are most often spread through contaminated water. Water purification methods have nearly eliminated these diseases in many countries, yet they remain a serious threat in many areas of the world. Outbreaks of intestinal illnesses, including those on cruise ships, are often caused by viruses (norovirus and rotavirus) that are spread through food by infected food handlers. Fecal contamination of food is the most common source of infection by the hepatitis A virus. Parasites can also be transmitted through food and water. Cryptosporidium parvum, Entamoeba histolytica, and Giardia intestinalis (also known as G. lamblia) are all protozoa that cause severe diarrhea. While rare in areas with good sanitation, these illnesses are still extremely common worldwide. It is estimated that 10 percent of the world’s population and 2 to 3 percent of the US population are infected with E. histolytica, which causes amebic dysentery. However, only between 10 and 20 percent of those infected will develop symptoms.

Direct contact transmission. Some pathogens are so sensitive to the environment outside the human body that they cannot survive long enough to be transmitted by casual contact. These organisms must be transmitted from one person to another directly—through the exchange of body fluids during sexual contact, blood transfusion, birth, or breastfeeding. Bacterial infections that are transmitted through sexual contact include syphilis (Treponema pallidum), gonorrhea (Neisseria gonorrhoeae), and chlamydia (Chlamydia trachomatis). A number of pathogenic viruses are transmitted through direct contact. These include human immunodeficiency virus (HIV); hepatitis virus B, C, and D; and herpes simplex virus (HSV). Other pathogens are transmitted directly through contact with the skin, often entering through a wound or break in the skin. Staphylococcal infections (Staphylococcus aureus) are transmitted in this manner. Antibiotic-resistant forms of staph infections are now common among athletes and are spread during contact sports and in locker rooms.

Prevention

Public sanitation programs have had a profound impact on the incidence of contagious diseases in developed countries. Public health measures to prevent the spread of waterborne and foodborne illnesses are generally quite effective. These measures include water purification and sewage treatment, waste removal, and enforcement of regulations to promote food safety during production and preparation.

A dramatic example of an effective public health program comes from data on typhoid fever in Philadelphia during the early twentieth century. In the ten years following the introduction of filtration and chlorination of the city’s water, the number of cases of typhoid fever dropped steeply from nearly ten thousand cases each year to just more than one hundred cases. This result is a heartening reminder that the spread of contagious diseases can be controlled.

Vaccination is the most effective method of preventing a variety of contagious diseases. Modern innovations in molecular biology have enabled the development of new vaccines that provide coverage against more diseases. Despite the availability of a range of vaccines, many adults in developed countries are not effectively immunized. In some cases, this is because immunity from their childhood vaccines has faded or may not have been very effective to begin with. In other cases, adults may not be aware that new vaccines are available or that they need to be immunized against different organisms as they age. Vaccination rates for children and adults in developing countries are low because of financial barriers and a lack of infrastructure.

Certain measures for disease prevention are up to each person to adopt. Sexually transmitted diseases can be prevented by using condoms during intercourse. Handwashing is an effective way to avoid infection by organisms spread through respiratory and oral-fecal routes. One should wash their hands before and after handling food, after using the toilet or changing diapers, after sneezing or coughing, and before and after treating a wound. (Soap and warm water should be used to scrub hands for a minimum of twenty seconds. If water is not available, alcohol-based hand sanitizer is another good option.) One should often clean kitchen countertops with soap and water. A disinfectant that destroys pathogenic organisms, such as 95 percent isopropyl alcohol, should be used occasionally in kitchens and bathrooms. One should avoid antibacterial soaps, however, because they are not more effective at killing bacteria than regular soap and can lead to the development of drug-resistant bacteria.

Emerging and Reemerging Infectious Diseases

Emerging infectious diseases are diseases discovered that were previously unidentified. Other types in this category are those found outside of their normal geographic areas. Oftentimes, travelers to foreign areas become infected with these diseases and transport them back to their home countries. Examples of emerging infectious diseases include HIV infections, SARS (severe acute respiratory syndrome), West Nile virus, dengue fever, the Zika virus, COVID-19, and MERS (Middle East respiratory syndrome).

Even after a disease is believed to have been eradicated or significantly reduced, many are capable of making a dramatic reappearance. These are known as reemerging diseases. This category of disease is affected by human behaviors. An example is when public health measures that successfully contain the disease are relaxed. Another cause is new strains of the disease that emerge or the current strain developing resistance to antibiotics or other treatments.

Examples of reemerging diseases include malaria, cholera, tuberculosis, and gonorrhea. In June 2022, an unvaccinated patient in New York City became the first reported case of polio in the United States in over a decade. Polio was eradicated in the US in 1979, but due to an increasing number of individuals choosing not to receive the vaccine, herd immunity protection eventually began failing.

Other reemerging diseases reported in the 2010s and 2020s include scarlet fever, measles, tuberculosis, whooping cough (Pertussis), Mpox (formerly monkeypox), Marburg virus disease (MVD), and the fungal infection Candida auris (C. auris). Vaccines exist to prevent or lower the severity of whooping cough, Mpox, measles, and tuberculosis.

Impact

Contagious diseases have been intimately associated with human life throughout history. In fact, human history has been shaped by contagious diseases. These diseases continue to cause suffering, disability, and economic hardship for millions of people in both developed and developing nations. Every year, nearly one-quarter of all deaths worldwide (about 12 million) are caused by contagious diseases.

Also, all nations face the financial burden of disease prevention and treatment. Widespread contagious illnesses can be so costly that they hamper economic development and political stability in developing countries. The COVID-19 pandemic in the US cost approximately $14 trillion from 2020 through 2023. In 2024, the Centers for Disease Control and Prevention estimated that the healthcare cost of treating the US's six most common antimicrobial-resistant infections was $4.6 billion annually. The global economic burden of major infectious diseases has been estimated at around $8 trillion annually.

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