Send an email to my boss Joan telling her that I won't be coming to work today because I'm sick. Keep it short but make it sound like I'm very ill and won't be in for the rest of the week. Sign it off with the name John and make it sound professional.
Dear Joan,

I am writing to inform you that I will not be attending work today. I have developed a severe illness and am very much unable to work. In fact, my doctor has advised me not to attend work for the rest of the week. Apologies for my sudden absence.

Best wishes,
John 