What are 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 disease 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, however, are contagious. An example of an
infectious but not contagious disease is malaria,
which is caused by the protozoan Plasmodium falciparum, which is
transferred from one person to another by the bite of a mosquito. Direct contact
with the infected person or with that person’s body fluids will not spread this
disease. A notorious 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 microorganisms that are pathogenic.
Contagious diseases are caused by viruses, bacteria, fungi, protozoa, and
helminths. The specific biology of each organism determines what part of the body
it will infect, what symptoms it will cause, how it will be 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 potentially 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 of spread of many bacterial,
viral, and parasitic diseases. 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 U.S. population are
infected with E. histolytica, which causes amebic
dysentery.

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
breast-feeding. Bacterial infections that are transmitted through sexual contact
only 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 (Staphyloccocus 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 food-borne 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. Recent 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 that are spread through respiratory and oral-fecal routes. One should wash his or her 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 counter tops 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.

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 the economic development and political stability of developing nations.

Bibliography

Flint, S. J., et al. Pathogenesis and Control. Vol. 2 in Principles of Virology. 3d ed. Washington, D.C.: ASM Press, 2009. Discusses the general principles of infection by viruses, the mechanisms of infection and its spread in populations, immune responses, vaccination, and antiviral drugs.

Gilligan, Peter H., M. Lynn Smiley, and Daniel S. Shapiro. Cases in Medical Microbiology and Infectious Diseases. 3d ed. Washington, D.C.: ASM Press, 2003. The most common pathogens are presented by affected organ systems. Includes medical cases with descriptive examples of symptoms associated with infection by particular organisms.

Madigan, Michael T., and John M. Martinko. Brock Biology of Microorganisms. 12th ed. Upper Saddle River, N.J.: Pearson/Prentice Hall, 2010. A standard microbiology textbook for undergraduate college students, with detailed descriptions of cell structures and clear illustrations. Includes evolutionary perspectives and covers pathogenesis.

Percival, Steven L., et al. Microbiology of Waterborne Diseases. San Diego, Calif.: Academic Press/Elsevier, 2004. Major pathogenic waterborne microorganisms are described in terms of physiology, reproduction, clinical features and treatment of infection, and survival in the environment.

Ryan, Kenneth J., and C. George Ray. Sherris Medical Microbiology: An Introduction to Infectious Diseases. 5th ed. New York: McGraw-Hill Medical, 2010. Standard medical microbiology textbook covering pathogen biology, clinical presentation and diagnosis, treatment, and epidemiology.

Shannon, Joyce Brennfleck, ed. Contagious Diseases Sourcebook. Detroit: Omnigraphics, 2010. For general readers, this sourcebook provides information about the transmission and treatment of contagious diseases. Includes facts about prevention, self-care, and drug resistance.

Wilson, Michael. Microbial Inhabitants of Humans: Their Ecology and Role in Health and Disease. New York: Cambridge University Press, 2005. Biology and ecology of microorganisms indigenous to the human body. Describes environmental features of all organ systems in the human body and discusses microorganisms that reside in each.