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: Apparent clinical success in the treatment of severe adenovirus disease is limited to a few case reports and small series .
### Output Text:
Apparent clinical success in the treatment of severe adenovirus disease is limited to a few case reports and small series . 