Differences Between Bacteria and Viruses

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Bacteria are single-celled, prokaryotic microorganisms that exist in abundance in both living hosts and in all areas of the planet (e.g., soil, water). By their nature, they can be either “good” (beneficial) or “bad” (harmful) for the health of plants, humans, and other animals that come into contact with them. A virus is acellular (has no cell structure) and requires a living host to survive; it causes illness in its host, which causes an immune response. Bacteria are alive, while scientists are not yet sure if viruses are living or nonliving; in general, they are considered to be nonliving.

Infections caused by harmful bacteria can almost always be cured with antibiotics. While some viruses can be vaccinated against, most, such as HIV and the viruses which cause the common cold, are incurable, even if their symptoms can be treated, meaning the living host must have a strong enough immune system to survive the infection.


Learn the basic differences. There are key differences between bacteria and viruses in size, origins, and effects on the body.

• Viruses are the smallest and simplest life form; they are 10 to 100 times smaller than bacteria.
• Bacteria are single-celled organisms that can live either inside or outside other cells. They can survive without a cellular host. Viruses, on the other hand, are only intracellular organisms, meaning that they infiltrate the host cell and live inside the cell. Viruses change the host cell’s genetic material from its normal function to producing the virus itself.
• Antibiotics cannot kill viruses, but can kill most bacteria, with the exception of bacteria that have become resistant to the antibiotic. Misuse and overuse of antibiotics have led to antibiotic resistance. Antibiotics are becoming less effective against potentially harmful bacteria. Gram-negative bacteria are highly resistant to treatment with antibiotics, but can be killed by some.

Recognize the differences in reproduction. Viruses must have a living host to multiply, such as a plant or animal. Meanwhile, most bacteria can grow on non-living surfaces.

• Bacteria have all the “machinery” (cell organelles) needed for their growth and multiplication and usually reproduce asexually.
• By contrast, viruses generally carry information – for example, DNA or RNA, packaged in a protein and/or membranous coat. They need the host cell’s machinery to reproduce. The “legs” of a virus attach onto the surface of the cell and then the genetic material contained inside the virus is injected into the cell. Put differently, viruses are not really “living”, but are essentially information (DNA or RNA) that float around until they encounter a sufficient host.

Determine whether the organism has a beneficial effect in the body. Though it may seem hard to believe, many, many tiny organisms live within (but are distinct from) our bodies. In fact, in terms of pure number of cells, most people are roughly 90% microbial life and only 10% human cells. Many bacteria exist peacefully with our bodies; some even perform very important tasks, like making vitamins, breaking down waste, and making oxygen.

• For example, much of the process of digestion is done by a type of bacteria called “gut flora.” These bacteria also help maintain pH balance in the body.
• While people are familiar with “good bacteria” (such as gut flora), there are also “good” viruses, such as bacteriophages, that “hijack” the bacteria’s cellular mechanisms and cause cell death.Researchers from Yale have designed a virus that may help defeat brain tumors.Most viruses, however, have not been proven to perform any functions that are beneficial to humans. They typically only cause harm.

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