Why was viola disguised as cesario

In William Shakespeare's comedy, Twelfth Night Viola took a disguise after the shipwreck. As she founds herself alone in Illyria and knew that his brother Sebastian might be drowned she thinks that how to survive. She gets to know about the place and there is a vacancy at Duke's place. More importantly, that is only for a male. Keeping these things in her mind she has decided to take a disguise in order to survive in Illyria as Cesario.