Why are all Viruses pathogenic?

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Why are all Viruses pathogenic?

Postby Event Horizon on May 22nd, 2018, 2:25 pm 

https://www.livescience.com/48218-ebola ... fever.html

I can't think of one single virus that benefits its host in any way. Yet more kill the host, although bacteria can do that too. It's also the case that in Southern India that bats spread a deadly and previously uncommon virus that can infect humans, swine and other animals.

https://t.co/u4CZ79yNT9

Could CRISPr help us design anti-viral viruses in time to have a real impact?
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Re: Why are all Viruses pathogenic?

Postby wolfhnd on May 22nd, 2018, 3:18 pm 

Gene transfer?
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Re: Why are all Viruses pathogenic?

Postby Serpent on May 22nd, 2018, 4:01 pm 

Because they are missing most of the organs of 'proper' living cells. They can't reproduce without taking over a host cell and disabling its own replication capability.
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Re: Why are all Viruses pathogenic?

Postby doogles on May 22nd, 2018, 5:07 pm 

Serpent's answer seem mostly 'spot on' to me'

A couple of comments though. We usually don't look for and identify viruses unless they are causing a disease. So in one sense, every time we identify a virus, it's BECAUSE it is pathogenic.

Fruit bats in Australia have been shown to carry Hendra virus and Lyssavirus, both of which are pathogenic to humans and horses in particular, as well as a couple of other species. The Hendra virus has claimed a few people, including veterinarians who have to take great care now when attending sick horses. The Lyssavirus is apparently very similar to the Rabies virus.

It is possible to attenuate viruses so that they become non-pathogenic and act as vaccines. Pasteur did it back in the late 19th century with the Rabies virus, and I still have a problem getting my head around his persistence in discovering it. He was able to get sterile samples of a rabid dog's brain into sterile containers and preserve them for some time. He took a one-day-old sample and injected it into a healthy dog and produced rabies. This mystifies me a bit because the incubation period of rabies can be up to 6 months. Then he took a two-day-old sample and so on up to 11 days. The 11-day-old sample did not produce rabies. When he susequently challenged this dog with virulent virus, it turned out to be immune. He had attenuated the virus.

I would have given up on the idea after the first few days. That's a bit of irrelevent trivia. But I admire his persistence.

A second case that comes to mind is that of the Distemper virus of dogs. Somebody discovered in the 1950s (I think) that they could culture the virus on hen eggs. And after 40 subcultures on eggs, the virus was still alive but it no longer caused distemper. This attenated live Distemper virus was our main vaccination technique for some decades.

Obviously these attenuated viruses have lost their ability to penetrate and destroy host cells. It seems most probable to me that they are treated as foreign proteins when they enter the host and that antibodies are produced against them simply on that basis.

There is such a thing as cross-immunity where one virus that is relatively non-fatal can produce immunity to a pathogenic virus. The first official vaccine was the use of cow pox virus, which produced harmless lesions on the hands of people who milked cows, to protect against smallpox. Human measles vaccine does not affect dogs in any way, but protects against distemper. I think an attenuated measles vaccine is now the vaccine of choice for dog distemper.

I'm not sure where this places the statement that "All viruses are pathogenic". Maybe there are stacks of non-virulent viruses in our microbiomes and that if we are not too fussy about our hygiene when we are young, that we attain cross-immunity to many pathogenic viruses.

Just some thoughts ....
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