Home » Pet Health » Canine Viruses and Infections » What are Viruses?


Viruses are microscopic infectious agents that can replicate or multiply only when they are inside the living cells of an organism. They can infect a variety of living organisms such as bacteria, plants, humans, and animals (including dogs and cats). Commonly known human viruses include mononucleosis, chickenpox, whooping cough, and the virus that causes cold sores. In dogs, you may be familiar with parvovirus and distemper virus; in cats, you may have heard of feline leukemia virus. Learning how a virus works can help you understand what’s happening in your pet’s body when they have a viral infection.

How does a virus infect a cell?

A virus uses the following steps to infect a cell:

    1. Entry – The virus may enter the body through your pet’s eyes, nose, ears, mouth, or breaks in the skin.
    1. Attachment – The virus recognizes and binds to a cell by attaching itself to receptor molecules on the host cell surface.
    1. Infection – After attachment, the virus infects the host by injecting its genetic material into the host cell.
    1. Multiplication – The virus reprograms the host cell to produce multiple copies of itself.
    1. Spread – The host cell dies, and the viral copies are released, which spread throughout the body, infecting more cells. At this stage, your pet might start to feel sick.

Viruses may also remain hidden or dormant inside the host cells for long periods, without any symptoms. The immune system does not detect these viruses, and they can remain hidden for years before activating and replicating.

Regular check-ups with your vet will help you identify if your pet has a viral infection. If your dog or cat is carrying a virus, your vet can guide you on treatment and precautionary measures.

The Pet Cancer Foundation’s Website Editorial team is comprised of veterinarians, veterinary oncologists, and veterinary technicians, as well as scientific writers and editors who have attained their PhD’s in the life sciences, along with general editors and research assistants. All content found in this section goes through an extensive process with multiple review stages, to ensure this extended resource provides pet families with the most up-to-date information publicly available.

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Last Updated: July 20, 2022

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The following sources were referenced to write the content on this page:

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Cohen, FS 2016, ‘How viruses invade cells’, Biophys Jl, vol. 110, no. 5, pp. 1028-1032.

Lodish, H, Berk, A, Zipursky, SL, Matsudaira, P, Baltimore, D & Darnell, J 2000, ‘Viruses: structure, function, and uses’, Molecular Cell Biology, 4 edn, WH Freeman.

Martella, V, Elia, G & Buonavoglia, C 2008, ‘Canine distemper virus’, Vet Clin North Am Small Anim Pract, vol. 38, no. 4, pp.787-797

Morales-Sánchez, A & Fuentes-Pananá, EM 2014, ‘Human viruses and cancer’, Viruses, vol. 6, no. 10, pp. 4047-4079.

Munday, JS, Thomson, NA & Luff JA 2017, ‘Papillomaviruses in dogs and cats., Vet J, vol. 225, pp. 23-31.

Pollock, RV & Coyne MJ 1993, ‘Canine parvovirus’, Vet Clin North Am Small Anim Pract, vol. 23, no. 3, pp. 555-568.

Szymonowicz, KA & Chen J 2020, ‘Biological and clinical aspects of HPV-related cancers’, Cancer Biol Med, vol. 17, no. 4, pp. 864-878.

Trovato, M, Sartorius, R, D’Apice, L, Manco, R & De Berardinis P 2020, ‘Viral emerging diseases: challenges in developing vaccination strategies’, Front Immunol, vol.3, no. 11, pp. 2130-2150.

The Pet Cancer Foundation’s medical resource for pet owners is protected by copyright.

For reprint requests, please see our Content Usage Policy.