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: Given the seriousness and increasing prevalence of adenovirus disease in certain hosts , especially children , a large , multicenter clinical trial of potentially useful anti - adenoviral therapies , such as intravenous ribavirin , is clearly required to demonstrate the most effective and least toxic therapy .
### Output Text:
Given the seriousness and increasing prevalence of adenovirus disease in certain hosts , especially children , a large , multicenter clinical trial of potentially useful anti - adenoviral therapies , such as intravenous <span class="chemical">ribavirin</span> , is clearly required to demonstrate the most effective and least toxic therapy . 