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How Do Viruses Make Us Sick?

Pfizer Vaccines
Pfizer UK

8 November 2021

COVID-19 has put viruses front and centre, but what exactly are they?


Viruses are groups of nucleic acid—DNA or RNA —that are surrounded by a protein shell1 known as a capsid.2 As one of the most abundant life forms on earth,2 viruses are everywhere; experts estimate that they are 10 times more numerous than bacteria.3 And while individual viral particles, called virions,1 are too small to be seen under a microscope,3 when grouped together they can do enormous damage to people and animals.


How do viruses enter the body?

Viruses cannot reproduce by themselves. In fact, they are barely living when outside of a host.2 But these parasites are well designed to infiltrate host cells and reproduce plentifully.

First, a virus finds an access point on the surface of a host’s body, usually in the respiratory tract, including the mouth and nose.4 For pathogens such as rhinoviruses, coronaviruses, and influenza viruses, this is the most common route.5 Microbes such as enteroviruses often get in through the gastrointestinal tract6 when a person is eating or drinking contaminated food; human immunodeficiency virus (HIV), hepatitis, and herpes may gain entry through the urogenital area during sexual activity.7 Viruses can also enter through the surface of the eyeball, and skin that has an abrasion or injury;4 they cannot get past intact skin.8 Mucous linings in the throat, gastrointestinal tract, and genital tract provide protection against viruses, but they can still make their way through mucus.9


How viruses travel through the body

Once a virus enters a host’s body, it moves along the surfaces of cells until its proteins begin to bind with receptors on the cells. The virus and the cells then combine, allowing the virus's DNA or RNA inside the virus to enter the cells, where it begins to reproduce.1 If enough viral matter penetrates the host cells without being deactivated by a host’s immune system or other antiviral mechanism, infection begins rapidly.5

The replicating virus may stay at the entry site or it may spread to other cells and tissues in the body, often by entering the bloodstream. The bloodstream allows the virus to have a far reach inside the body.5 Viral infections can permeate their way into the nervous system either by directly infecting the nerve endings of the system, or by infecting other cells in the circulatory system that could make their way to the nervous system.10 Subsequently if the body cannot contain the spread of the infection on a local basis, it could have the adverse effect of spreading to other vital organs and causing damage elsewhere.10  


How we respond to viruses

The immune system jumps into action in response to the injury of these bodily cells, causing symptoms such as fever and chills. While we sometimes worry about running a fever, an elevated temperature generally is considered a protective response that works to destroy the invading virus.11 And while chills—which typically occur along with fever—may make you feel cold, they also happen because your core temperature is raised in order to fight off your infection.12

If we have previously been vaccinated against a virus, our body is better equipped to fight that virus when it encounters it again. The immune system remembers how it responded to the harmless form in the vaccine and replicates that process against the harmful version of the virus.13




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  9. Harvard University, Science in the News. All About That Mucus: How it keeps up health – March 2018. Available at: [Last accessed August 2021]
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  11. Healthline. A Guide to Viral Fevers – 13 July 2018. Available at: [Last accessed September 2021]
  12. UPMC Health Beat. Why Do I Have the Chills? – 31 August 2016.  Available at: [Last accessed September 2021]
  13. British Society of Immunology. How vaccines work. Available at: [Last accessed July 2021]


PP-VAC-GBR-1774 / Sept 2021