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Pathogens and the Spread of Disease


Pathogens, or infectious agents, cause infection and lead to disease. The four most common pathogens are viruses, bacteria, fungi, and parasites. Pathogens cause infection when they invade a host and make a home for themselves where they normally do not live. The infection becomes a disease when it causes irregular conditions and negative effects to its new home.

A scanning electron micrograph of the COVID-19 virus colored in yellow

Credit: The National Institute of Allergy and Infectious Diseases' Rocky Mountains Laboratories


Viruses are non-living beings made of DNA (deoxyribonucleic acid) or RNA (ribonucleic acid) that need other cells to reproduce. This becomes a concern for humans when they use our cells to replicate.

A scanning electron micrograph of E. coli bacteria

Credit: Michigan State University


Bacteria are small, single-cell organisms living almost everywhere on Earth. They are not all necessarily bad. Healthy bacteria inside us help our digestive and immune systems. Some bacteria become a concern when they move somewhere new, like to humans, and start causing disease. This happens when bacteria multiply and release toxins into our bodies and overwhelm our immune systems.

A colored scanning electron micrograph of the fungus Aspergillus fumigatus conidiophores

Credit: Eye of Science / Science Photo Library


Fungi are everywhere—indoors, outdoors, and on human skin. There are millions of fungal species, such as yeast, mold, and mushrooms. About 300 species cause infection and disease. This occurs when their spores get inside a host, like human lungs, and begin to grow.

A scanning electron micrograph of Trichuris muriss, a parasitic worm

Credit: Toby Starborg, Uta Rössler, Allison Bancroft, Richard Grencis


Parasites are organisms that need a host to survive and reproduce. Hosts unwillingly provide parasites a place to live and eat. They also move parasites from place to place, introducing them to new hosts to spread and continue their lives.

The Spread of Disease

Words like outbreak, epidemic, pandemic, and zoonotic diseases sound scientific, but defining them can be tricky. The use of these terms can be political and strategic since they carry nuanced meanings. They conjure different feelings and imagery in each person. But, what are some general definitions of these terms?

A view from Big Hill Park in Beloit, Wisconsin, overlooking the Rock River and fall foliage

Credit: Travel Wisconsin 

Zoonotic Diseases

Most of the diseases causing concern for humans are zoonotic diseases—diseases that come to humans from animals.

Zoonotic diseases have been around for millions of years and will be around as long as there are humans to infect. Most of the time zoonotic diseases are present within us and our communities without our knowledge. Other times these diseases stay, spread to other humans, and cause more noticeable, severe illnesses.


Before pandemics there is always an outbreak. An outbreak happens when a small group of people, animals, or plants gets sick. Outbreaks can be traced to a single event or exposure, such as spoiled food at a family picnic or a town’s contaminated water source.


From outbreaks, an infection or disease can become an epidemic.


The word epidemic dates to the 8th century BCE, when it referred to someone in their own country. By the 5th century BCE, epidemic referred to physical symptoms of an illness at specific places and times. After the discovery of bacteria in the 19th century, epidemic was used to refer to specific diseases. Today, epidemic also refers to there being more cases than expected during the spread of a disease.


Pandemics are an epidemic occurring worldwide, crossing international boundaries, and affecting a large number of people. Pandemics have little to do with severity and more to do with the novelty and spread of a disease.

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