According to my mum: “if you even miss a single day they throw the entire jury out and have to restart the whole court case again so that the new jurors can hear all the evidence”. I feel that would make longer cases exponentially impractical.
I can’t find anything about this on the internet, other than for someone asking this question in America.
Standard number in criminal trials is 12, and the judge can choose to have up to an additional 3 as spares if they think it’s necessary. See here: http://www.austlii.edu.au/cgi-bin/viewdoc/au/legis/nsw/consol_act/ja197791/s19.html
I believe they’d just postpone precedings until the juror is well (as maniacalmanicmania said was their experience). I’m pretty confident all jurors would have to hear all evidence.