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: Seventy - five human immunodeficiency virus ( HIV ) - infected patients with CD4 + cell counts < 500 / mm3 were randomized to receive either ZDV ( 500 mg daily ) alone ( group I , n = 38 ) or in combination with folinic acid ( 15 mg daily ) and intramascular vitamin B12 ( 1000 micrograms monthly ) ( group II , n = 37 ) .
### Output Text:
Seventy - five human immunodeficiency virus ( HIV ) - infected patients with CD4 + cell counts < 500 / mm3 were randomized to receive either <span class="chemical">ZDV</span> ( 500 mg daily ) alone ( group I , n = 38 ) or in combination with <span class="chemical">folinic acid</span> ( 15 mg daily ) and intramascular <span class="chemical">vitamin B12</span> ( 1000 micrograms monthly ) ( group II , n = 37 ) . 