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: 6 ng / ml , which were similar to those found in the serum ( parallel concentration time curves ) .
### Output Text:
6 ng / ml , which were similar to those found in the serum ( parallel concentration time curves ) . 