Contact UsInvestorsCareersMediaScienceContact Us
HomeNewsNews & Featured Stories How Do Viruses Make Us Sick?


How do viruses make us sick?

Pfizer Vaccines

Pfizer UK

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

How Do Viruses Make Us Sick?

Viruses are groups of nucleic acid—DNA or RNA —that are surrounded by a protein shell1 known as a capsid.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.And while individual viral particles, called virions,are too small to be seen under a microscope,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


  1. Cohen, F.S. (2016). How Viruses Invade Cells Accessed March 2023.

  2. Stanford Medicine Scope. What’s a Virus, Anyway? Part 1: The Bare-Bones Basics Accessed March 2023.

  3. National Human Genome Research Institute. Virus Accessed March 2023.

  4. Drexler M, (2010). What You Need to Know About Infectious Disease Accessed March 2023.

  5. Racaniello, Vincent, Columbia University. Viral Pathogenesis Accessed March 2023.

  6. MedicineNet. Enterovirus D68 (EV-D68): Symptoms and Treatment Accessed March 2023.

  7. State Department of New York, Department of Health. Disease That Can be Spread During Sex Accessed March 2023.

  8. Miller, Chris Prof. Registered Dental Hygiene Magazine. Infection Control: The skin is an excellent barrier to infection Accessed March 2023.

  9. Harvard University, Science in the News. All About That Mucus: How it keeps us healthy Accessed March 2023.

  10. Koyuncu OO, Hogue IB, Enquist LW (2013). Virus infections in the nervous system Accessed March 2023.

  11. Healthline. A Guide to Viral Fevers Accessed March 2023.

  12. UPMC Health Beat. Why Do I Have the Chills? Accessed March 2023.

  13. British Society of Immunology. How vaccines work. Accessed March 2023.

PP-UNP-GBR-4229 / March 2023
Science Products Responsibility PartnershipsCareers Privacy Statement Terms of Use Contact UsSitemap Copyright © 2017-2024 Pfizer Limited. PP-UNP-GBR-8108 / January 2024. All rights reserved. Registered in England and Wales No. 526209. Registered office: Ramsgate Road, Sandwich, Kent CT13 9NJ. VAT No. GB201048427. This website is intended for the general public in the UK.