Can viruses be toxic?
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)