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Bacteria can produce toxins like endotoxins and exotoxins. In diseases like cholera or tetanus they can harm infected people due to these toxins. Now, although viruses are much smaller and are dependent on the host cell for their production, are there DNA sequences in viruses which can produce similar toxins, or are viruses made of something that is toxic?

If not, can viruses be seen as causing an allergic reaction because like common allergies they don't harm directly, only our immune system generates the problems?

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In general, DNA that encodes toxins can be incorporated in a viral genome and thus it can be expressed in the infected host inducing toxicity. Here an example: http://www.sciencedirect.com/science/article/pii/S1438422104000542

Also, viruses can trigger allergic reactions (http://www.ncbi.nlm.nih.gov/pubmed/21980825) or promote them (http://erj.ersjournals.com/content/19/2/341).

Finally, there are some reports about viruses causing "with symptoms that mimic allergies" (https://www.youtube.com/watch?v=iDF07R1XHSk)

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    $\begingroup$ "The genes encoding Stx are encoded in the genome of heterogeneous lambdoid prophages (Stx-converting bacteriophages; Stx-phages)" You say in general but is Stx the only known viral toxin? If I understood the article well it comes originally from a bacteriophage but are there also viruses that could programme our own cells by producing toxins? $\endgroup$ – Marijn Mar 24 '16 at 19:40
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    $\begingroup$ @Marijn Ofc. not, there are many toxic viruses, e.g. ebola has (at least one) toxin too. ncbi.nlm.nih.gov/pubmed/15596847 $\endgroup$ – inf3rno Mar 25 '16 at 5:35
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In addition to @alec_djinn's answer, viruses can also indirectly be toxic by causing lysis in cells that themselves have toxic components. The presence of bacterial endotoxins in poorly filtered solutions of early attempts at phage therapy is probably the canonical example of this.

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In theory, there's no reason why a virus couldn't encode bacterial toxins. In practice, that doesn't happen much because viruses have no incentive to sabotage their host cells in that manner - there's largely no fitness advantage, all it would do is make the viral genome larger and shut down the host cell preventing further replication.

Even if a virus specifically wanted to shut down its host cells, it's more efficient to spawn large numbers of copies of itself and exhaust the host cell's resources.

in short:

virus produces toxins in host cell -> host cell dies -> no replication opportunity and virus gets trapped in the cell

virus produces large numbers of copies of itself -> host cell runs out of resources and dies -> numerous replications and avoid getting trapped

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Yes viruses can import toxin encoding genes, at least in humans and in plants.

With Ebola a protein called Delta-Peptide is released in large amounts by the virus genome, and it attaches to cell walls and makes them more permeable. it does this to the cell wall, and some researches have described it as ebola toxin. enter image description here http://jvi.asm.org/content/early/2017/05/18/JVI.00438-17 https://www.sciencedaily.com/releases/2015/01/150121103304.htm

Toxin from a maize virus: http://www.sciencedirect.com/science/article/pii/0006291X87903792 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2171720/

Conversely, plants generally use alkaloids as toxins. Alkaloids are a large family of N containing organic molecules, for example those found in Datura, Angel of Death mushroom, Cyanide, etc. and I found no evidence that viruses make us produce alkaloids. Scientists research on the anti-viral abilities of some alkaloids.

Tonins in venoms are more often protein based... Tangentially to your question, here's a page on the diversity of venom toxins for background. https://en.wikipedia.org/wiki/Venom#Diversity

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