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: From these results , we conclude that ribavirin has an antiviral effect in advanced cases of AHF , and that anemia , the only secondary reaction observed , can be easily managed .
### Output Text:
From these results , we conclude that <span class="chemical">ribavirin</span> has an antiviral effect in advanced cases of AHF , and that anemia , the only secondary reaction observed , can be easily managed . 