What Are Pathogens?

Pathogens are agents that live in a host, such as a human, and are responsible for causing infections or diseases in that host. Pathogens can be microorganisms — bacteria, viruses, fungi or protozoa — or other organisms such as parasites, worms and even infectious proteins known as prions.

Pathogens make up a small minority of the microorganisms and organisms humans encounter throughout the course of life. The human microbiota consists of over 10,000 species of microorganisms, the vast majority of which are not harmful and have a mutually beneficial relationship to the host. On occasion, however, disruptions to the microbiota can allow some usually harmless microorganisms to flourish and cause an infection or disease. On other occasions, a pathogen from an external source may invade the body and cause infection or disease. For these microorganisms, a human host offers a moist, warm and nutrient-rich environment for harmful pathogens to grow and flourish.

Types of Pathogens

Here are some common types of pathogens that can cause infection and disease in humans:

Viruses: Viruses are microorganisms made up of nucleic acid and covered with a protein coat. Some viruses also have a lipid (fatty) outer coat. These are called enveloped viruses. Those that do not have this lipid outer coat are called non-enveloped viruses. Viruses are able to multiply only within the living cells of their host. Examples of infections caused by viruses include influenza (influenza A and B viruses) and the common cold (rhinovirus).

Bacteria: These are microorganisms that have a DNA-containing nucleus surrounded by a membrane or cell wall. The composition of the cell wall differs between bacteria. They have a more complex structure than viruses and can multiply outside the host if they are able to access food and nutrients. Bacteria such as Salmonella enterica and Escherichia coli can cause foodborne illnesses, while Staphylococcus aureus can cause skin infections.

Fungi: Fungi are single- or multi-celled organism that are able to survive outside a host. They absorb food and nutrients produced after the secretion of digestive enzymes into their surroundings. Examples of infections caused by fungi include athlete’s foot (Trichophyton mentagrophytes) and yeast infections (Candida albicans).

Prions: These are infectious agents made up entirely of proteins and capable of transmitting their infectious properties to other prion proteins. Prions can cause serious infectious diseases, including bovine spongiform encephalopathy (mad cow disease) and variant Creutzfeldt-Jakob disease (vCJD).

Protozoa: Protozoa are single-celled organisms that exhibit animal-like behaviors, such as the ability to move through water, and can prey on other microorganisms. They are able to exist outside the human host but can also be parasitic in nature when they enter the human host and multiply, causing infections and illnesses. Common infections caused by protozoa include malaria and giardia.

How Are Pathogens Transmitted?

There are a number of ways pathogens are spread and transmitted between people. Direct transmission occurs when there is physical contact between people, while indirect transmission involves contact with a non-human pathogen reservoir. Which pathogens are spread by which routes depends on the makeup and properties of the pathogens. Some common methods of transmission of bacteria, viruses and fungi include the following:

Aerosol transmission: Pathogens can be carried in small airborne particles called droplet nuclei. These particles are typically less than 5 microns in diameter and as such can be suspended in the air for several hours. When inhaled in sufficient amounts, they can cause an infection. Tuberculosis is an example of an infection transmitted by this route.

Droplet transmission: In people with infections of the upper respiratory tract, the act of sneezing and coughing expels oral or nasal droplet secretions containing the pathogens responsible for the infection. These droplets are relatively large and can stay suspended in the air for only a few minutes. The droplets can be inhaled or can enter the oral cavity via this indirect route and cause an infection. Examples of infections transmitted by the droplet route include influenza, the common cold and chicken pox.

Fomite transmission: Inanimate surfaces can become contaminated if they are touched by an infected person or if infected droplets land on them. The pathogens can then be transmitted to the hands of someone touching the surface, and then to the mouth or nose, potentially causing an infection. This is an example of indirect transmission. Pathogens can live for days to weeks on common surfaces and still cause infections when picked up by people and transferred to the body via the nose and mouth.

What Are Bloodborne Pathogens?

Bloodborne or bodily fluid transmission: The transmission of pathogens through bodily fluids such as blood is a common concern with pathogens such as HIV and hepatitis B. Transmission can occur when the infected bodily fluid from one person enters another person through cuts and abrasions, needlesticks or mucous membranes. Surfaces containing bodily fluids can also play a role in this method of transmission. Transmission of bloodborne pathogens can be either direct or indirect.

Bloodborne pathogens are infectious microorganisms that are primarily spread through contact with infected human blood and bodily fluids. They are known to cause a variety of diseases and illnesses common in humans. Some examples of bloodborne pathogens include HIV, hepatitis B virus, and hepatitis C virus.1 These types of pathogens can be encountered anywhere, including the home and workplace, but the risk of transmission is especially high in healthcare settings in which healthcare workers deal with sharp needles and other instruments that puncture the skin. Healthcare facilities and most workplaces are required to comply with the OSHA Bloodborne Pathogens Standard, which outlines protocols to protect both workers and patients from exposure and possible infection.2

What Are Foodborne Pathogens?

Foodborne pathogens are spread through contaminated food or drink, or by infected people, and can cause mild, severe and even deadly infections. Common foodborne pathogens include Salmonella enterica, Campylobacter jejuni and norovirus, but unspecified agents, or unidentified pathogens, are the largest cause of foodborne illnesses each year.

Two of the most detrimental foodborne illnesses identified by public health systems are salmonella and norovirus. Salmonella is the cause of the most deaths associated with foodborne illness in the United States annually. Norovirus is common in healthcare and long-term care settings. It can be spread through contaminated food and surfaces, and by sick people who shed the virus in their feces, contaminate their hands and then do not wash them thoroughly. While most infections are mild and resolve quickly, infections are more severe in the elderly, young children and those with pre-existing illnesses, who are less capable of effectively fighting off illness.

What Are Airborne Pathogens?

Airborne pathogens are infectious microorganisms that are commonly spread through tiny airborne particles called droplet nuclei. Typically less than 5 microns in diameter, droplet nuclei can remain suspended in the air for several hours once they are exhaled. When droplet nuclei are exhaled by someone with an infection and then inhaled by someone else, they can cause infection and disease. Tuberculosis is an example of an infectious disease that is spread through the airborne route. Viruses that infect rodents can also be spread through the airborne route, carried through tiny dust particles. Hantavirus, which is shed in the urine and feces of mice, can be carried through the air on dust particles and can cause an infection in humans if inhaled.

Pathogen Survival on Surfaces

One route of pathogen transmission is through contact with contaminated surfaces. Many pathogens can survive for long periods of time — from several hours to months — outside the human body, commonly on surfaces. For example, while the influenza virus can persist on hard, nonporous surfaces such as ceramic, plastic and steel for 8–48 hours depending on the environmental conditions,3 norovirus can persist for up to 28 days.4 In the case of norovirus, only a few virus particles can cause an infection, so the survival of even a few particles can present a transmission risk. Some bacteria such as Staphylococcus aureus can persist for up to seven months, while the spore form of Clostridium difficile can persist for up to five months.

The long survival times of common pathogens on surfaces is an issue because of the potential for transmission from surfaces to healthy people, even long after they have become contaminated. This is a concern in communal spaces such as offices, schools and athletic facilities, especially during flu season and outbreak situations, and makes effective environmental cleaning and disinfection critical to prevent the transmission of pathogens.

The Burden of Infectious Disease

From health and economic perspectives, the burden resulting from illnesses caused by infectious microorganisms in the United States is huge. Annually, foodborne pathogens such as Salmonella enterica, norovirus and Campylobacter jejuni are estimated to cause illness in one in six Americans — around 50 million, and result in 128,000 hospitalizations and 3,000 deaths. In 2014 the financial cost of these illnesses was estimated at $15.6 billion annually.5

The burden from influenza and the common cold is also significant. Annually, people in the U.S. experience 500 million common colds annually. This results in 70 million lost work days and 189 million lost school days, resulting in direct and indirect costs of $40 billion.6 Seasonal influenza causes over 31 million outpatient visits, 200,000 hospitalizations and a significant number of deaths. This puts a significant burden on the health system and healthcare workers, and the cost to the economy has been estimated at $87 billion.7

Healthcare-associated infections (HAI), infections acquired while in healthcare facilities, are also a significant contributor to the burden of infectious illness. In 2011 it was estimated that there were approximately 722,000 cases of healthcare-acquired infections recorded in U.S. acute care hospitals. Of these cases, around 75,000 died during their hospitalization.8 In long-term care facilities, it has been estimated that there are 1–3 million infections annually, resulting in 380,000 deaths.9 HAIs are detrimental to the patients, staff and the reputation of healthcare facilities striving to provide high-quality care. HAIs can affect any patient, but those with weakened immune systems, those undergoing intensive medical procedures, and the elderly, such as those living in long-term care facilities, are at particularly high risk.

Antibiotic resistance has been an increasing concern with regard to foodborne pathogens such as Salmonella enterica and Campylobacter jejuni. It is considered a serious threat to public health by systems such as the Centers for Disease Control and Prevention (CDC). In fact, these two pathogens alone cause an estimated 410,000 antibiotic-resistant infections every year in the United States.10 While the sicknesses caused by these pathogens are normally easily treated with antibiotics, the microbes have evolved over generations to resist these life-saving drugs, forcing drug companies and government organizations to scramble to find new treatment options.

Clorox Professional Products Help Prevent the Spread of Pathogens

Norovirus Prevention: Norovirus can survive for up to 28 days on common surfaces in the environment. Only a small number of virus particles are required to cause an infection, so contaminated surfaces can contribute to the spread of the virus. Learn how effective environmental cleaning and disinfection is critical to eradicate the virus and prevent it from spreading.

Clostridium difficile Infection (CDI) Prevention: According to the CDC, some 450,000 Americans acquire CDI annually. In healthcare settings, transmission via contaminated surfaces is a particular concern and highlights the need to implement effective infection control protocols and practices. Learn more about Clostridium difficile and how surface disinfection can reduce the risk of transmission.

Methicillin-resistant Staphylococcus aureus (MRSA) Prevention: MRSA infections affect people in community and healthcare settings. MRSA skin infections can be spread through contaminated surfaces and items used by those with infections, such as towels. Hospital-acquired MRSA infections are more serious and can result in long, expensive hospitalization. Learn how effective disinfection is essential to prevent the spread of MRSA.

Salmonella Prevention: A salmonella infection can be spread by the consumption of contaminated water or food. Learn more about how to prevent salmonella outbreaks in your facility.

Influenza Prevention: Influenza virus particles can survive on hard surfaces for up to 48 hours after contamination, making effective environmental disinfection an important strategy to prevent transmission. Learn more about how to prevent the spread of seasonal flu.


1. Occupational Safety and Health Administration. Bloodborne pathogens and needlestick prevention. Accessed December 19, 2017.
2. Occupational Safety and Health Administration. Occupational safety and health standards.  Accessed December 19, 2017.
3. Kramer A, Schwebke I, Kampf I. How long do nosocomial pathogens persist on inanimate surfaces? A systematic review. BMC Infect Dis. 2006;6:130.
4. Kim AN, Park SY, Bae SC, Oh MH, Ha SD. Survival of norovirus surrogate on various food-contact surfaces. Food Environ Virol. 2014;6(3):182-8.
5. USDA: U.S. foodborne illnesses cost more than $15.6 billion annually. Food Safety News. Accessed February 8, 2017.
6. Fendrick AM, Monto AS, Nightengale B, Sarnes M. The economic burden of non-influenza-related viral respiratory tract infection in the United States. Arch Intern Med. 2003;163(4):487-94.
7. Molinari NA, Ortega-Sanchez IR, Messonnier ML, et al. The annual impact of seasonal influenza in the US: measuring disease burden and costs. Vaccine. 2007;25(27):5086-96.
8. Magill SS, Edwards JR, Bamberg W, et al. Multistate point-prevalence survey of health care–associated infections. N Engl J Med. 2014;370(March 27): 1198-1208. doi:10.1056/NEJMoa1306801
9. Centers for Disease Control and Prevention. Nursing homes and assisted living (long-term care facilities). Accessed February 9, 2018.
10. Centers for Disease Control and Prevention. Food Safety: Antibiotic Resistance and Food Safety., Accessed December 19, 2017.