In the sentence extracted from biomedical literature, identify all the disease entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="disease"> to denote a disease.

### Input Text: The Cox proportional - hazards model was used to adjust for covariates including age , sex , tumor site , degree of tumor differentiation , and use of adjuvant therapy .
### Output Text:
The Cox proportional - hazards model was used to adjust for covariates including age , sex , <span class="disease">tumor</span> site , degree of <span class="disease">tumor</span> differentiation , and use of adjuvant therapy . 