When writing a card to someone who is sick, the most important thing is to be encouraging. Tell the person how loved she is, how she is in your thoughts, and that if she needs anything, she can count on you.
In a card to a sick person, it is best to let her know she is loved and being thought of. She should know that if she needs anything, she can call on you. Offer to bring her dinner or clean her house, if she lives close by. Otherwise, just let her know that she is being thought about.
One of the best things to write in a card to a sick person is "I love you". Regardless of whether a person has the flu or a fatal disease, knowing that she is loved is one of the best gifts she can receive. It allows her to know that she has support and care through her sickness.
There are no rules as to what to write in a card to someone who is sick, but being encouraging is the best approach. Make her hopeful of recovery, let her know she matters, and wish her all the best.
Focus on the positive and avoid negativity. Let her know that she is strong, and wish her a speedy recovery.