The difference between bacteria and viruses

Bacteria and viruses can make you sick - they don't have much more in common. We explain the difference between the pathogens from size to control with drugs

In contrast to viruses (right), bacteria (left) can multiply themselves. A virus needs a host to reproduce

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We all know them from everyday life: bacteria and viruses. We know that they are invisible to the human eye, occur almost anywhere, and can make us sick. They come in many different forms, and many of them existed at a time when there were no larger living beings on earth. But what is the difference between a virus and a bacterium?

The size: bacteria are much larger than viruses

The easiest way to distinguish bacteria and viruses is by their size. "Bacteria are up to a hundred times larger than viruses," explains Dr. med. Clemens Fahrig, Chief Physician and Medical Director of the Clinic for Internal Medicine at the Evangelical Hospital Hubertus in Berlin. Most bacteria are about 0.6 to 1.0 micrometers (µm) in diameter. They can still be seen with a normal light microscope. The much smaller viruses, on the other hand, can only be seen under an electron microscope.

The blueprint: Viruses are more simply structured

"The main difference is the blueprint," says Fahrig. Viruses and bacteria also differ in their "anatomy". "Bacteria, for example, have a real cell wall and an internal structure," explains the specialist in internal medicine. Inside the wall lies the cytoplasm, the ribosomes and the genetic material of the bacterium.

The cytoplasm is the basic structure of the cell and metabolic processes take place in it. The ribosomes help with protein synthesis and with the reproduction of the bacterium. In addition, many bacteria carry one or more flagella, which are used for locomotion. Some species have so-called pili, which help to attach to other bacteria, surfaces or cells.

Viruses are more simply structured. They mostly consist only of their genetic material, which is enclosed in a shell of proteins, the capsid. Some viruses also have a virus envelope consisting of a lipid bilayer.

Multiplication: Viruses need a host

Bacterial cells, like human cells, usually multiply through cell division. Before a bacterial cell can divide, it copies its genetic material. Then the bacterium constricts itself in the middle. A mother cell becomes two daughter cells, which in turn can divide themselves.

Viruses cannot multiply on their own. Since they have no cytoplasm and no ribosomes, they can neither copy their genetic material nor produce their own shell. Viruses therefore attack foreign cells, the so-called host cells, into which they smuggle their own genetic information. The genetic information of the virus "programs" the genetic material of the host cell so that it produces many more viruses. The new viruses leave the host cell either by exocytosis, which is what is called discharge from the cell, by budding or by lysing. Viruses are pinched off with parts of the cell during budding, and the host cell's membrane is dissolved during lysis.

Bacteria are living beings, viruses are not

The way of life and thus the metabolism of bacteria are also very different. There are bacteria that need oxygen for their metabolism and some for which oxygen is poison. Some bacteria need light to exist, others certain chemicals, such as sulfur.

Viruses do not have their own metabolism. Therefore, in contrast to bacteria, they are not counted among the living beings, at best among the borderline forms.


bacterium virus
Size: 0.1-700 microns
20-300 nanometers


single-celled beings

own metabolism and

own cell

not a living being

without metabolism and

without cell

Propagation: Cell division Host cell

Composition of bacteria and virus


Structure of a bacterium

A bacterium is a single-celled living being. In addition to their genetic makeup (DNA), all bacteria have a cytoplasm, a cytoplasmic membrane and ribosomes. This allows the bacteria to operate their own metabolism.

Some bacteria also have a cell wall membrane, flagella for locomotion or pili for attachment to surfaces.

Build a Virus

In our case, the virus has a protein envelope (capsid) as well as a virus envelope (lipid double layer). This is not the case with all viruses.

Viruses with a virus envelope are usually sensitive to solvents for fat, including soap. Therefore hand washing helps.


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How bacteria and viruses make you sick

Bacteria and viruses make us sick in different ways. Viruses can, for example, destroy cells in our body during their reproduction process. Or the body's own defense cells eliminate the cell infected by the virus. For example, bacteria can cause illness through their metabolic products. Some of these are toxic to humans.

Different treatment: antibiotics only help with bacteria

"Antibiotics influence and attack the structures of the bacteria, such as the cell wall, and can lead to the death of the bacteria," says Fahrig. Because the cell wall of bacteria has a different structure than the cell membrane of the body's cells, antibiotics only attack bacteria and not body cells. Some antibiotics do not kill the bacteria at all, but only prevent them from multiplying. "However, bacteria are independent cells that can adapt to other environmental conditions. Therefore, bacteria can also develop antibiotic resistance," notes Fahrig.

Since viruses do not have their own metabolism or cell wall, antibiotics are powerless. However, there are also agents that prevent viruses from multiplying. In the case of many viral infections such as colds, treatment is limited to therapy that does not fight the virus itself, but alleviates the symptoms of the disease. The body's defenses have to take care of the rest on its own.

Antivirals inhibit the multiplication of viruses

Antivirals are drugs that use different mechanisms to inhibit the reproduction of some viruses. They are used for certain viral diseases. "The virus can be prevented from spreading in the body," says the specialist in internal medicine. The drug often attacks not only the viruses, but also the body's own cells. Also, effective drugs are not available for all diseases caused by viruses.

Antivirals have different points of attack in the multiplication stages of a virus. Some drugs prevent the virus from docking or entering the host cell. Others, on the other hand, disrupt the production and composition of the genetic material or the envelope.

Vaccinations can protect against some bacteria and viruses

A vaccination prepares the immune system to defend itself against the pathogens. There are vaccines against certain bacterial and viral diseases. A vaccine contains either weakened pathogens or only harmless parts of the pathogen. The immune system recognizes the foreign structures and forms antibodies against them. If the same type of pathogen enters the body after the vaccination, the intruder can usually be rendered harmless quickly. "This has been possible for decades with many viral diseases such as polio," explains Fahrig. "In the case of HIV or hepatitis C, however, this has not yet been successful because of the complicated properties of the viruses."

In addition, some viruses change their outward appearance very quickly. That makes vaccine development difficult. That is why, for example, a new vaccination serum has to be developed every year for the flu pathogen.