Skip to main content

Differences Between Viruses and Bacteria

  • Author:
  • Updated date:

Infection is generally defined as the invasion of the body by microorganisms causing disease and harm. Infectious diseases are caused by viruses and bacteria, but did you know that these are two different things?






Generally smaller, cannot be seen through a common microscope

Larger than viruses, can be observed microscopically

Host requirement

Needs a host cell to reproduce

Does not need to invade a host cell to reproduce

Type of infection

Systemic, spread throught the body

Usually localized, but can spread systemically if not treated

Host relationship

Harmful most of the time

Sometimes beneficial, sometimes harmful


Antiviral drugs, antibiotics have no effect




Viruses are microscopic pathogens that infect living cells and tissues. They are the smallest kind of microbe, with the size ranging from around 20-200 nanometers, around 35 times smaller than a human red blood cell and around 100th the size of a regular bacteria.

Viruses are not living things. They are complex molecules of proteins and genetic material but they do not have their own cell structure. Viruses can not replicate without infecting a living cell. Unlike bacteria that has everything it needs to reproduce, viruses need to use a living cell's organelles (cell parts that are basically it's organs) in order to replicate. Viruses infect all living things, including fungi and even bacteria. Viral mode of transmission includes droplet contact, sexual and parenteral contact and the fecal-oral route.

There are different types of viruses, all of them with their respective host ranges. There are some viruses that can infect more than one type of organism, as seen in the avian flu for example. Viruses produces disease usually by killing off enough cells to cause damage, or by disrupting the body's homeostasis, the system in which the body maintains all of its functions. Unlike the bacteria, most diseases caused by viruses are systemic; they affect the whole body. An example of this would be the flu which, while usually infecting the upper respiratory tract, affects the body through fatigue and fever.

Treatment of viruses is difficult. Since the virus invades the host cell, it's difficult to kill without harming the host cell itself. Antibiotics have absolutely no effect on viruses. There had been some progress, however, with antiviral drugs. These medicines introduce fake genetic molecules to the virus to stop it from reproducing. These drugs are commonly used on more serious infections such as HIV and Hepatitis. There is usually no need to take antiviral drugs for the less serious infections as the body's immune response can usually fight it off in itself.

Vaccines are our front line defense against virus. Vaccines introduce the virus to the host in a non-harmful way, so that if the time comes and the host gets infected, the immune system response would be quicker which ultimately prevents disease. Vaccines are purely preventive. It has no effect if the host is already infected.



Bacteria are larger than viruses. They come in a variety of shapes, typically spheres and rods. They are living things, complete with organelles and a 'skin' called cell membrane. Some bacteria are capable of movement through tail like structures called flagella. Bacteria usually reproduce via binary fission, a form of asexual reproduction where the bacteria replicates its DNA then divides itself into two identical cells. Unlike viruses, bacteria do not need a host cell (although they still need nutrients) to reproduce.

Harmful bacteria are referred to as pathogens. These pathogens cause disease that usually start in a specific location but when left untreated, can cause septicemia (the blood becomes infected and unusable by the body) which leads to shock and ultimately death. Most bacterial infections produce pus, a substance containing dead white blood cells. White blood cells, or leukocytes are our body's response to the bacterial infection. They engulf the bacteria and produce chemicals that kill any other bacteria that resists being engulfed.

Not all bacteria are harmful. Under normal circumstances, our body has a wide variety of bacteria referred to as human normal flora. These bacteria actually contributes to body functions, like digesting nutrients and protecting us by preventing other harmful bacteria from using our bodies as host.

Antibiotics are used to treat bacterial infections. There are two types of antibiotics: bactericidal antibiotics which kills bacteria, and bacteriostatic antibiotics which only prevent its reproduction and growth and must work with the immune system in order to get rid of the infection. There is an inherent danger in using antibiotics, particularly concerning antibiotic misuse. If an antibiotic regimen is stopped before the prescribed date, the few remaining bacteria which are not enough to cause symptoms or disease can develop resistance to the antibiotic.This resistance can be passed on to the next generation of bacteria when it reproduces. This is why it is very important to obtain and follow a physician's prescription when using antibiotics. Bacterial resistance is a very significant problem in disease control. It can lead to multi-drug resistant strains of bacteria which can be very difficult to treat. Excessive use of antibiotics can also kill of the normal body flora which can lead to opportunistic infections by fungi and other bacteria.

Bonus: Fungi and Parasites

Aside from bacteria and viruses, there are two other common microbes that we commonly encounter, the fungi and the parasites


Fungi are multi-celled organisms that are similar to plants, but have their own different kingdom. They include infections such as athlete's foot and candida. Common organisms such as molds and mushrooms are also fungi. They are treated by anti-fungal drugs, antibiotics usually have no effect on them.


Parasites are also multi-celled organisms that have a much more complex cell structure than bacteria. Parasites are usually larger than most bacteria, and can easily be seen under a microscope and sometimes, through the naked eye. The most common mode of transmission of parasites is through ingesting contaminated water or food.

From the author

I hope I answered your question, or the question you didn't know you have before you've read this article. Microbiology could be a fascinating subject to read upon. It's quite amazing how something so little can affect our lives in the biggest possible way. Please feel free to ask me anything and I'll try my best to answer you. Thank you for reading!


Miss on July 21, 2019:

Very setisfiying information.thanks

mr dookie on April 29, 2019:

lol are you still leaving comments on peoples comments when you made this article years ago

Mahfuza Amin on January 23, 2017:

very informative and easy to understand

troll lol lol on January 12, 2017:


Catalin on November 23, 2016:

Thank you very much for sharing with us your knowledge on this subject!

sara on September 11, 2015:

write the answer tomorrow in 4 in pm

sara on September 11, 2015:

i actully need to no how we can explain the differences between bacteria virus microbes and parasites

Paz (author) on February 16, 2014:

You are welcome, tino!

tino on February 15, 2014:

thank you soo much clean and simple

Paz (author) on July 13, 2013:

You're welcome, glad I can help!

rwhite on July 12, 2013:

Thank you, have an infection control assignment to write and you've described it really clearly - v helpful :)

Paz (author) on May 29, 2013:

And thank you for reading!

William F Torpey from South Valley Stream, N.Y. on May 29, 2013:

Thank you, Pazthelobster. I've been wondering about the difference between viruses and bacteria for decades. I don't have to wonder any more!