1982
To determine whether the risk of HIV seroconversion among daily smokers of crack cocaine changed over time , we used Cox proportional hazards regression and divided the study into 3 periods : May 1 , 1996 - Nov .
 Disease: HIV seroconversion; Chemical: crack cocaine
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Disease, Chemical 
Text: {0} 
Answer:
 Disease: HIV seroconversion; Chemical: crack cocaine