Transmission Basics guide for patients

Transmission Basics

Reviewed By:
David Slotnick, M.D.


Transmission is the act or process of spreading a disease. Diseases occur when a person becomes infected with a disease-causing microbe called a pathogen. There are a variety of different types of pathogens, including:

  • Bacteria
  • Viruses
  • Parasites, including protozoa (single-celled organisms) and helminths (multicellular worms)
  • Fungi

When a pathogen enters the body, it multiplies. Sometimes the immune system is able to fight off the infection. But other times, the pathogen multiplies too quickly for the immune system to eliminate it. This results in disease.

Diseases can be spread in a variety of ways, including:

  • From person to person. This is the most common method by which disease is spread. It can involve respiratory, sexual, mother-to-child (vertical) or surface contact (fomite) transmission.
  • From an animal or insect to a person. Diseases are often spread from animals and insects to people through a bite. Infected animals and insects may harbor pathogens in their saliva, which are then transmitted to people through a bite if the saliva enters the bloodstream. Animals or insects that are responsible for transmitting diseases are called vectors.
  • From eating or drinking contaminated food or water. In the United States, foodborne and waterborne diseases are not as common as in some developing countries. However, some pathogens can be carried in food or water. The salmonella bacteria, which cause salmonellosis, are among the most common foodborne bacteria.
  • From a blood transfusion or organ transplant. Since the 1980s, blood and organ tissue has been routinely screened for the presence of certain diseases such as hepatitis, HIV and syphilis. This has significantly reduced the instances of transmission of those diseases from blood transfusions or organ transplants. There are, however, rare cases of other diseases such as West Nile virus and cytomegalovirus being transmitted in this way.

It is impossible to completely avoid pathogens, but there are steps that people can take to reduce their risk of infection. These include practicing good personal hygiene, ensuring sanitary food preparation, maintaining a strong immune system and keeping vaccinations up to date.

About transmission

Transmission is the act or process of spreading a disease. Infectious diseases are so named because they are easily transmitted to others. Diseases can be transmitted from person to person, from animal/insect to person, or from infected water or food. Sometimes, diseases are also transmitted through blood transfusions or organ transplants, although this is becoming rarer in the United States.

Diseases are spread by infection with a disease-causing microbe called a pathogen. Pathogens can include:

  • Bacteria. These single-celled microbes are so small they can only be seen through a microscope. They are among the oldest living organisms on the planet and are very resilient. Bacteria have been known to survive in intense heat and cold, and they tend to become resistant to medication easily. Many types of bacteria thrive in the human body, although most are harmless. Less than 1 percent of all bacteria cause disease in humans.

  • Viruses. A virus is a tiny infectious agent with genetic material (DNA or RNA) that enters the body and takes over the function of the cell to which it attaches. It then directs the infected cell to produce copies of the virus and slowly infects large parts of the body. Most viruses cause disease – many of the most serious diseases in humans, such as AIDS and Ebola hemorrhagic fever, are caused by viruses.

  • Parasites. These organisms use the body as a host as they feed from its nutrients. Parasites include certain protozoa (single-celled organisms) such as malaria, which is transmitted via mosquitoes. Insects or insect-like animals (e.g., ticks) may carry parasitic protozoa and are known as vectors when they do so. Parasites also include multicellular worms called helminths. Most helminths infect the intestinal tract, where they can lay eggs and survive by consuming the nutrients in the body. Some helminths can grow up to 25 feet (7.6 meters) and are notoriously difficult to treat. Tapeworms, for example, can break off into sections, each of which can form a new tapeworm.

  • Fungi. Most people are familiar with fungus as mushrooms, yeast and mold. However, there are thousands of different types of fungi, many of which live in the human body. Some fungi are harmless, while others cause diseases. Diseases caused by fungi are called mycoses

When a pathogen enters the body, it attaches itself to a section of tissue (e.g., the intestines or the lungs) and multiplies. In some cases, the immune system is able to fight off the infection before it gets to the stage where it causes the body harm. However, if the pathogen multiplies faster than the immune system can fight it, then it results in disease. The symptoms of a disease normally reflect the site of the infection. For example, bacteria from contaminated food that enter the gastrointestinal system often cause nausea, diarrhea and vomiting. People who contract pneumonia by breathing in a fungus experience a tightness in the chest and persistent cough as the pathogen multiplies in the lungs.

Sometimes physicians use the phrase community-acquired to describe an infection. This means that the infection was transmitted in the community (for example, at school or in the workplace). Most infections are community-acquired. This is because people have contact with a wide range of pathogens in their daily lives, only some of which may progress to an illness.

Physicians may also refer to a nosocomial infection. Nosocomial means that it was acquired while the patient was in a hospital. Nosocomial infections can be more severe than community-acquired infections because patients’ immune systems are often compromised by whatever is causing them to be hospitalized in the first place. In addition, many of the pathogens that are common in hospitals are already resistant to some of the drugs used to treat them and, therefore, may be harder to treat.

In recent years, new infectious diseases that can affect humans have emerged, such as severe acute respiratory syndrome (SARS) and monkeypox. Although these diseases have the potential to be highly contagious, outbreaks in the United States are rare. For example, at the height of the SARS outbreak, there were only eight confirmed cases in the United States, according to the U.S. Centers for Disease Control and Prevention (CDC). In each of the eight confirmed cases, the infected people had traveled to areas of the world where SARS was more common.

Some more easily transmitted diseases include:

  • Influenza. According to the CDC, influenza is believed to infect 5 percent to 20 percent of the American population each year, making it one of the most common infectious diseases in the United States. Of those infected, more than 200,000 people are hospitalized and 36,000 die from the flu and its complications, according to CDC estimates.

  • Salmonellosis. As one of the most common foodborne illnesses, the salmonella Bacteria infect approximately 40,000 people every year, according to the CDC. However, it is believed that many more cases go unreported.

  • Sexually transmitted diseases (STDs). Although STDs are among the most easily preventable diseases, they are also among the most commonly transmitted. The CDC estimates that 2.8 million people are infected with chlamydia and 45 million people have come into contact with the herpes simplex virus, which causes genital herpes.

Person-to-person transmission

The most common way that diseases are transmitted are from person to person. This can happen in different ways, including:

  • Respiratory transmission. This involves the transmission of pathogens from the respiratory system through coughing, sneezing or otherwise expelling breath into the air. Some pathogens are classed as airborne, which means that they may be present in very small droplets in the air and can survive suspended in the air for long periods of time. Diseases that are transmitted via airborne pathogens include measles, smallpox and tuberculosis. Other pathogens are transmitted in larger droplets that are only expelled short distances and fall from the air relatively quickly. Examples of diseases that are transmitted in large particle droplets include whooping cough, influenza and pneumonia. In general, airborne pathogens are transmitted more rapidly and infect more people than those that are transmitted in large-particle droplets.
  • Sexual transmission. Sexually transmitted diseases (STDs) are diseases that are spread through sexual activities, including vaginal intercourse, anal intercourse, oral sex (mouth-to-genital contact) or any other form of intimate genital-to-skin contact. STDs can travel from person to person in bodily fluids including semen, vaginal fluids and blood. Some can enter the body through tiny cuts or tears in the mouth, anus or genitals. STDs are passed more often from men to women than the reverse because the exposed area is larger in women. In addition, the vagina acts like a reservoir that prolongs contact with infectious fluids. Microscopic injuries during intercourse are also more common in women than in men.
  • Mother-to-child (vertical) transmission. Many pathogens can be transmitted to unborn children through their mothers. Some pathogens, such as HIV or the toxoplasmosis parasite, can be transmitted through the placenta and infect the child. Other diseases, especially those that are present in the cervix or genitals such as gonorrhea and chlamydia, can be transmitted to the child as it passes through the birth canal during childbirth. Additionally, HIV and, according to some studies, West Nile virus can be transmitted to a child through breast milk.
  • Fomite transmission. A fomite is any surface onto which pathogens may be spread and infect people. This can include the clothing or bedclothes of an infected person, surfaces (such as doorknobs or counters) that may have come into contact with an infected person, or money, which may have come into contact with thousands of people and thousands of pathogens. Different pathogens can survive (and remain infectious) for different lengths of time on such objects and surfaces.

Animal/insect-to-person transmission

Some diseases can be spread from insects or animals to humans. This type of transmission can happen when a person travels to a country where animals or insects carry diseases that have not been experienced by the traveler’s immune system. However, some animals and insects in the United States can also transmit potentially fatal diseases to humans, such as rabies and West Nile virus.

Animals and insects can transmit pathogens to humans in a variety of ways, including:

  • Animal bites. Animal bites, especially those that puncture the skin, are one of the most common ways by which animals transmit pathogens to humans. Bacteria, viruses and parasites are commonly present in the saliva of animals. Many of these are often harmless for the carrier, but can cause disease in humans. For example, the macaque monkey is a carrier of the herpes B virus. Despite the fact that the monkey itself is not affected by the virus, it can transmit it to humans by biting or scratching. The herpes B virus affects the central nervous system and, if left untreated, causes death in 80 percent to 90 percent of human cases, according to the National Institutes of Health (NIH).

    Another disease caused by animal bites is rabies. Rabies is a rare disease that can be spread by numerous types of infected animals, including dogs, bats and raccoons. People who have been bitten by an animal that is rabid or suspected to be rabid should receive a rabies vaccination. People who have had contact with bats are also encouraged to receive a vaccination for rabies.
  • Bites from arthropods (insects and insect-like animals). Most diseases that are spread to humans from arthropods come from mosquitoes and ticks that can transfer pathogens from one creature to another (e.g., from an infected bird to a human). This occurs when the vector feeds from the blood of the infected party and then becomes infected itself. The pathogen travels to the salivary glands of the infected insect, where it can then be transmitted the next time the insect or tick bites.

    One of the most common insect-to-human diseases is malaria, which each year infects roughly 300 million to 500 million people worldwide, according to the World Health Organization (WHO). Although the disease is rare in the United States, there are about 1,300 cases of malaria reported in the United States each year, most of which originate in people who had visited countries with high malaria rates. The malaria parasite is transmitted to humans by female mosquitoes that become infected after feeding on an infected person.
  • Feces. In addition to bites, animals and insects can transmit diseases to humans by contaminating surfaces or food with their feces. Flies, in particular, breed in and feed on decomposing animal and plant matter, feces and the vomit of other animals. They can then contaminate food and surfaces as they land on them, in addition to defecating on surfaces themselves. Other animals and insects that commonly spread diseases through their feces are mice, cockroaches and some kinds of beetles.

Waterborne and foodborne transmission

There are numerous pathogens present in the food and water that people consume every day. Most of the time, the levels of contamination in food and water are too low to cause disease in people with healthy, functioning immune systems. However, in some cases, the number of pathogens is high enough to result in illness.  

The most common symptoms in food- and waterborne illnesses are nausea, vomiting and diarrhea as the pathogen enters the gastrointestinal system and multiplies there. However, microbes can also infect other body tissue or produce toxins that can be absorbed into the bloodstream.   

Waterborne diseases

In many countries, especially those in the developing world, waterborne diseases are among the biggest killers of people. In the United States, serious diseases caused by waterborne pathogens are rare. However, there are certain substances in water that can cause illness in some people. Water can become contaminated when chemicals are released into the water system or when the water is contaminated with the blood, feces or other bodily fluid of animals or humans. Some waterborne pathogens that may be present in water in the United States include:

  • Cryptosporidium. Protozoa that can cause gastrointestinal problems. Cryptosporidium may be found in drinking water and recreational water throughout the world. It is a common cause of waterborne disease of humans in the United States.
  • Giardia lamblia. This protozoan parasite is found in human and animal fecal waste, which may contaminate water sources. It is another common cause of waterborne disease in the United States, according to the Centers for Disease Control and Prevention (CDC).
  • Legionella. The bacteria that causes Legionnaire disease.

Foodborne diseases

There are more than 250 foodborne diseases, according to the CDC. The CDC also estimates that there are 76 million cases of disease caused by foodborne pathogens, which result in 5,000 deaths every year. Food can become contaminated in numerous ways. Some pathogens are present in the intestines of healthy animals and may infect other carcasses during slaughter, when intestinal contents come into contact with freshly slaughtered meat. Additionally, fruits and vegetables that are washed in contaminated water or grown in contaminated soil can carry pathogens on their skins.

The most common foodborne pathogens include:

  • Campylobacter. These bacteria are the most commonly identified bacterial cause of diarrhea in the world. They live in the intestines of birds and are frequently present in poultry meat. Campylobacter is most often transmitted to humans through undercooked poultry, or food that has been contaminated with raw chicken.
  • Salmonella. These bacteria are very common in the intestines of many types of animals, including birds, mammals and reptiles, and are easily spread to humans. The resulting illness, salmonellosis, can be especially serious in those with compromised immune systems such as elderly people, children and people with HIV/AIDS.
  • E. coli. Infection with the E. coli O157:H7 bacteria is one of the more severe examples of food poisoning common in the United States. The bacteria live in the intestines of cattle and can contaminate any kind of food that has come into contact with cow feces. It is most often transmitted to humans through undercooked ground beef but also may occur in unpasteurized milk or vegetables contaminated with feces.

People with HIV/AIDS or other conditions that seriously weaken the immune system are more susceptible to food- and waterborne pathogens than people with normal immune function. These people are advised to filter their tap water or use bottled water. They are also encouraged to cook all meat and poultry thoroughly and wash fruit and vegetables before cooking or eating them.

Other transmission methods

In rare instances, pathogens can be transmitted to humans through organ transplants or blood transfusions. During the process of organ or blood donation, participants are routinely screened for risk factors that may indicate the presence of an infectious disease. This may include questions related to medical history, foreign travel, sexual habits or drug use. In addition, all donations are screened for diseases such as HIV, hepatitis B and C, and syphilis. However, there are some diseases that can be transmitted by an organ transplant or blood transfusion, such as:

  • Cytomegalovirus (CMV). This virus is most often transmitted to infants from their mothers, but can also be transmitted through bone marrow and organ transplants. It rarely produces any symptoms but can cause serious illness in people with compromised immune systems.
  • West Nile virus (WNV). Although rare, there have been documented cases of West Nile virus being transmitted through organ transplants and blood transfusions. There is currently no routine test for WNV in blood or organs. However, donors are screened for risk factors.
  • Babesiosis. The Babesia parasite is most commonly transmitted by infected ticks and certain species of mice. Instances of transfusion-related transmission have also been reported.

Due to effective screening methods, transmission of HIV through organ transplants or blood transfusions is now rare. According to the Red Cross, the risk of contracting HIV through a blood transfusion is one in 1.5 million. Many people contracted HIV from blood transfusions or blood products received between 1977 and 1985, when testing of blood supplies for HIV began.  

People who have received organ transplants or blood transfusions outside the United States, especially in developing countries where screening methods are limited, may be at greater risk for contracting a disease from their transfusion.

Prevention methods

In today’s world, it is practically impossible to completely avoid pathogens. However, people can take steps to reduce their risk of contracting infectious diseases, including:

  • Personal hygiene. The most efficient way to prevent the transmission of many types of pathogens is to practice good personal hygiene. It is important that people wash their hands regularly, especially after visiting the bathroom and before eating or preparing food. Ideally, hands should be washed with warm water and soap for at least 15 seconds, although an alcohol-based sanitizer is a good alternative if clean water is not available.

    People who have been diagnosed with an infectious disease and those who are caring for people with infectious diseases should take extra precautions to prevent transmission. This includes washing clothing and bedding thoroughly and washing the hands after handling anything that may have been in contact with the blood, urine, stool and other bodily fluids of the infected person.

  • Sanitary preparation of food. Many cases of foodborne diseases can be prevented by taking suitable precautions during the preparation and cooking of food. Meat should be thoroughly cooked to an internal temperature of 160 degrees Fahrenheit (71.1 degrees Celsius), and poultry should be cooked to 165 degrees F (73.9 degrees C). Eggs should be cooked until the yolk is hard. Fruits and vegetables should be washed in warm water before consumption. Additionally, surfaces and utensils that have come into contact with raw meat should be disinfected before they are used again.

  • Safe sex. Taking appropriate steps to avoid contracting a sexually transmitted disease can prevent many types of infectious diseases. Preventive measures may include using condoms and dental dams, and avoiding risky sexual behaviors (e.g., having multiple sexual partners, having sex with people whose disease state is unknown).

  • Healthy immune system. One of the keys to effectively fighting infection is to maintain a healthy immune system. This can be achieved by eating a balanced diet rich in essential nutrients. Smoking and consuming excess alcohol should be avoided because these may also reduce the effectiveness of a person’s immune system. Another important element of maintaining a healthy immune system relates to the use of antibiotics. Antibiotics should only be prescribed for certain infections and should not be taken for a viral infection. If antibiotics are prescribed, they should be taken for the full course because pathogens may become resistant to antibiotics that are stopped before the immune system has eliminated the disease.

  • Vaccinations. Immunization is often the most efficient way to prevent many infectious diseases. Vaccinations are particularly important for people who may be encountering pathogens for the first time, such as infants and young children and people who are traveling to other countries.

Questions for your doctor about transmission

Preparing questions in advance can help patients have more meaningful discussions regarding their conditions. Patients may wish to ask their doctor the following questions related to transmission:

  1. Are there any infectious diseases for which I am particularly at risk?
  2. What can I do to prevent transmitting my disease?
  3. What health precautions should I take before I travel?
  4. What health precautions should I take while I am traveling?
  5. Can I transmit my disease to my unborn child?
  6. Can my pet transmit diseases to me or my children?
  7. Are there any food preparation techniques that would reduce my risk of contracting a foodborne disease?
  8. If I am pregnant, are there any foods I should avoid because of the risk of foodborne diseases?
  9. Is it safe for me to donate blood if I have an infectious disease?
  10. If I have a weak immune system, is there anything I can do to reduce the risk of contracting a disease?
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