In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: There are no approved antiviral agents with proven efficacy for the treatment of severe adenovirus disease , nor are there any prospective randomized , controlled trials of potentially useful anti - adenovirus therapies .
### Output Text:
There are no approved antiviral agents with proven efficacy for the treatment of severe adenovirus disease , nor are there any prospective randomized , controlled trials of potentially useful anti - adenovirus therapies . 