Blood culture. Wat is tht all abt!

My father age 63. He had mild heart attack. We did anjio kept stem. After tht he was normal for ten days. Then he had stroke. Due to which he couldn't talk properly. He can walk talk but unable to spk. Now again he is getting fever very often. He is getting ecups even for three days continues. Ecups stops for five days again getting ecups continues for two or three days. Doc saying we need to do blood culture and find out why he is getting fever and ecups . Sometimes they say if people get stroke ecups wil come lik this. Some times they say it may be any infection due to which he gets fever and ecups. It would be great if Abu one can suggest . And say why they asking to do blood culture. Wat is tht all abt