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: Hepatitis B virus ( HBV ) infection , which causes liver cirrhosis and hepatocellular carcinoma , remains a major health problem in Asian countries .
### Output Text:
Hepatitis B virus ( HBV ) infection , which causes liver cirrhosis and hepatocellular carcinoma , remains a major health problem in Asian countries . 