What is a Slow Virus?

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  • Written By: wiseGEEK Writer
  • Edited By: O. Wallace
  • Last Modified Date: 07 November 2019
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Slow virus is becoming an outdated term for what is often called a prion disease. The basic concept is that an infectious agent enters the body but doesn’t at first manifest. Instead, months or years may pass before the slow virus emerges, and its emergence may be tragic and difficult because many of these disease types cannot be cured and result in degeneration and death. Examples of prion disease and slow virus include bovine spongiform encephalopathy (Mad Cow) and Creutzfield-Jacob disease.

The reason the slow virus is now referred to as a prion disease is due to theories about how these diseases get passed. They are not typical viruses, but instead are usually caused by body accumulation of very small cells called prions. Prions don’t produce as normal viruses do because they lack DNA. Instead it’s thought they replicate by converting normal prions found in the brain into abnormal ones that aren’t broken down in the regular fashion.

The slow virus is also challenging because it’s difficult to tell until symptoms begin that anyone is sick. People don’t have immune reactions to prions, so in early stages of “infection,” they seem perfectly fine and blood testing would appear to confirm that. Years can pass before abnormal prions have accumulated to such a degree that symptoms emerge.


In conditions like Mad Cow disease, cows tend to get sick through eating beef in their food that is contaminated with these virus-like prions. Development of Mad Cow symptoms like blindness and eventual death can actually move pretty swiftly for what is deemed a slow virus. Symptoms may still not be present, though, when cows are slaughtered, and the disease can be passed to humans who ingest meat from sick cows.

Sometimes a prion disease has an inherited aspect to it. People may inherit, presumably from someone who has abnormal prions, the prion’s ability to behave abnormally and convert healthy prions into ones that can steadily accumulate. It is thought that Creutzfield-Jacob disease is inherited in this fashion. As with mad cow, Creutzfield-Jacob is devastating, causing conditions like dementia, memory loss, speech loss, movement challenges and eventually death. As yet, no treatment exists that can adequately reverse the progress of this slow virus, though one is actively sought.

From time to time, the term slow virus may be used to express viral dormancy that may occur with other viruses that live in the body. The chicken pox virus can remain dormant in the body for five or six decades and then reemerge as shingles in older adults. It’s thought that expression of mononucleosis is not always immediate or that initial exposure to some herpes simplex viruses doesn’t always lead to immediate signs of infection. More often though, slow virus now refers to an infection with abnormal prions, resulting in a severe, but slow-progressing disease.


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