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: A grade 2 or 3 infection occurred in 16 % of patients , but no toxic deaths occurred .
### Output Text:
A grade 2 or 3 infection occurred in 16 % of patients , but no toxic deaths occurred . 