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: Although intravenous ribavirin was not effective for all children with severe adenovirus disease in this series or in the literature , therapy is unlikely to be of benefit if begun late in the course of the infection .
### Output Text:
Although intravenous <span class="chemical">ribavirin</span> was not effective for all children with severe adenovirus disease in this series or in the literature , therapy is unlikely to be of benefit if begun late in the course of the infection . 