It must be different by state? I was on a jury (civil trial), the alternates had to stay. I was sick at the time (bronchitis) and kept coughing and wheezing during testimony. I had to wave to the bailiff so I could leave the courtroom and finish my coughing spasm in peace (and get water). They took a break, and I found out when I came back the judge had asked the lawyers if they wanted to excuse me from serving (and bring in and alternate). The lawyers said no. :(
Do you like serving on the jury?