Effect of Viruses on Host Cells

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.

In this lesson we'll be exploring how viruses exert their detrimental effects on host cells. We'll go over the cytopathological implications, such as the latent and lytic cycle, as well as oncogenesis.

What Are Viruses?

Most of us have had a cold at some point in our lives. You have a stuffy nose, or a sore throat, or a cough. These illnesses are caused by viruses, or non-living infectious particles. They get into your body through physical contact, or even simply through the air. If you've ever been near someone who has a cough, you know how quickly a virus can spread.

But, why do viruses make us sick? Viruses infect our body and invade our cells. They take over normal cell functions and force cells to make more viruses. Viruses often kill host cells, which is where the sick feeling comes from. Your cells are dying and your body is mounting an immune response to try to get rid of the virus.

There are several different ways viruses cause problems, called cytopathology, or how disease progresses inside cells. Some viruses don't produce any effects for many years, while others cause extreme, fatal symptoms within hours. Other viruses have even been linked to cancer.

Latent Viruses

Although viruses like the common cold tend to affect us within days, some viruses take years to produce any symptoms. This delayed onset is called the latent cycle. One common example of a virus in the latent cycle is the human immunodeficiency virus (HIV). HIV is a virus spread by blood, semen or vaginal fluids. It infects white blood cells that are an important part of our immune system. People can go for many years without even knowing they are infected because HIV remains latent for much of the infection.

HIV viruses shown in green bud from an immune cell
HIV virions

When HIV enters immune cells, it starts by incorporating its DNA into the host cell DNA. The host cell doesn't realize that anything is wrong and reads the new DNA, producing new viruses. However, some HIV infected cells go into a latent period where they do not produce new viruses. HIV essentially hides inside the cells, which is how a person can go many years without realizing they have the disease.

Latent viruses like HIV also produce a problem for long term treatment. With certain anti-viral medication, doctors are able to decrease the viral load, or amount of HIV viruses in the body, to very low levels. But the latent cells are still around holding HIV DNA. When treatment is stopped, these cells start making new HIV and the infection comes back.

Lytic Viruses

Other viruses aren't so subtle. Ebola is a hemorrhagic fever virus that causes excessive bleeding, organ failure, and death in up 90% of patients. This virus is highly lytic, meaning that it causes host cell death upon infection. Although Ebola causes host cell lysis rapidly, viruses can cycle between latent and lytic cycles.

Ebola is able to infect many different types of cells in the body, which accounts for its ability to induce large scale organ failure. It starts by infecting several types of immune cells, which leads to both a decreased ability to fight the virus and large scale release of chemicals that cause inflammation in the body, fever, and a loss of integrity in the blood vessels.

When Ebola infects the cells that line blood vessels, called vascular endothelial cells, the cells start to pull apart from one another, leading to internal bleeding and eventually cell death. This combined with a decreased immune response and intense inflammation trigger organ failure quickly.

Ebola produces wide spread symptoms because it causes damage to many types of host cells
ebola symptoms

Oncogenic Viruses

Viruses in the latent portion of their lifecycle incorporate their DNA into host cell DNA. This can cause problems for the host cell, since DNA is the blueprint for everything the cell needs to make to do its job and stay alive. When a virus inserts its DNA, it can cause disruption in these instructions either through insertion or through the products the viral DNA produces.

Mutations, or changes to the DNA of a cell, can result in uncontrolled cell growth, also known as cancer. Viruses that lead to cancer are called oncogenic viruses. One commonly known oncogenic virus is human papilloma virus (HPV). Although there are over 200 different strains of this virus, only a few are considered high-risk for developing cancer, most commonly cervical cancer.

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