Difference Between Viral & Bacterial Infections

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

This lesson is on viral versus bacterial infections. In this lesson we'll cover what a virus is and how it is different from a bacterium. We'll also go over examples of viral and bacterial infections.

What Is an Infection?

Have you ever been to the doctor, maybe sick with the flu in the winter, and they tell you there is nothing they can do? You might wonder, why did I even come here and why can't I have antibiotics? Well, if you don't get antibiotics, the doctor has probably found you have a viral infection, not a bacterial infection. Doctors won't prescribe antibiotics for viral infections because they won't work! Although it seems like a pain when you're the sick patient, this practice is for a good reason for, and that's what we're going to learn today.

Virus Versus Bacteria

To get started, let's review the difference between a virus and a bacterium. Viruses are non-living, infectious particles. They require a host cell to survive as they are not alive on their own. Viruses are beyond microscopic and require a special type of microscope to be seen. Bacteria are much larger comparatively and are living, single cells. Some bacteria are good for us, and we need them to survive. Other pathogenic bacteria infect our bodies and make us sick. The diagram compares the two types of pathogens.

Bacteria are up to 50 times larger than viruses.
virus vs. bacteria

Viral Infections

So how do viral infections happen? Well, first a virus must enter the body through the airways, digestive or reproductive systems, or cuts in the skin. Once the virus has entered the body, it will make its way to a specific tissue. Both viruses and bacteria infect specific types of tissue. This is why you have specific symptoms for each infection. Once the virus has reached its destination, it binds, or attaches, to the host cell. The virus either enters the cell, or directly injects its genetic material, or instructions, into the cell. Once the virus has access to the cell, it hijacks the host cell's machinery to make more viruses. The host cell is a slave to the virus, creating more and more viruses that burst out of the host cell to invade other tissues. Sometimes, this is a rapid process and the host cells die quickly. Other times, the virus may wait or exit the cell slowly, prolonging the infection. The image shows a general example of viral replication. The specific method depends on the virus, but all viruses work similarly.

Viral infection
Viral replication

An example of a viral infection is human immunodeficiency virus (HIV), the virus that causes acquired immunodeficiency, or AIDS. HIV is transmitted through blood, semen, or vaginal secretions. It quickly invades the blood stream and attacks the host's immune cells. With a damaged immune system, the patient can no longer fight other diseases and often dies of secondary infections.

Bacterial Infections

Since bacteria are alive, they don't need to invade host cells specifically to replicate. Bacteria find the tissue they like to live in, similar to the virus. The bacteria may take up nutrients from the host tissue, secrete toxins that kill the host cells, or replicate inside the host cells, killing them as the bacteria break out. However, bacteria replicate on their own, unlike viruses that need the host cell to make more of them. In the diagram, we see the two ways bacteria can kill a host cell.

Bacteria can burst host cells or damage them with toxins.
modes of infection

An example of a bacterial infection is E.coli O157:H7, a particular strain of E.coli that causes food poisoning. Not all E.coli are bad for us, but this strain secretes a powerful toxin causing vomiting and diarrhea and can be fatal if not treated. The E.coli in the image is colored pink and the background is purple.

E.coli O157 caption=

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