I've been absent for work because if systemic viral infection. is my absence justifiable because of my diagnosis?how to explain it to my employer?

Depends: Beyond recognizing your justification, you'd have my appreciation for preventing a spreading to others. But unless your local laws are wiser than most, it will be up to the management as to how they react. Try to have a written note from the doctor and that will do the major part of an explanation. I hope you succeed or that if you don't you can challenge it or that you find a better job. Best!

Yes...: First of all if you don't rest during your illness you might get worse and as a consequence miss more days at work than you would if you rested in the beginning of the illness. Other reason not to go to work is to avoid giving the virus to other people there as well. Hope they understand and that you get better soon. Rest and drink plenty of fluids in order to recover quickly.