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: Illness occurred within 1 - - 9 weeks of commencement of therapy in 9 patients , the remaining 3 patients having received the drug for 13 months , 15 months and 7 years before experiencing symptoms .
### Output Text:
Illness occurred within 1 - - 9 weeks of commencement of therapy in 9 patients , the remaining 3 patients having received the drug for 13 months , 15 months and 7 years before experiencing symptoms . 