Your body needs more energy when you are ill to facilitate the recovery process which includes the production of white blood cells to help fight the infection. This shouldn't be limited to proteins though, carbs are also very important as they are provide high amounts of energy. Basically just making sure you eat a varied and balanced diet is more important when you are ill to make sure your body recieves the nutrients it needs to fight infection and heal itself. As it is having to work harder when you are ill it makes sense to eat more high energy foods. Micronutrients are also very important!

>Minerals such as selenium and zinc work to keep the immune system strong. These minerals are found in protein rich foods such as beans, nuts, meat, and poultry.[1]

although if your diet is varied you should be getting plenty of these anyway without needing to take supplements.

[1] http://www.webmd.com/cold-and-flu/flu-guide/what-to-eat-when-you-have-the-flu