Why does my husband want to make love when I am ill? Is he serious?

It might seem annoying but I reckon he just wants to be close to you despite you not feeling well. It is more desirable for some if the object of their pursuit is unavailable, in this case you being sick and feeling vulnerable could very well be arousing to your husband. I don't think it's anything particularly negative haha. Just tell your husband in a nice way that you love him but you don't feel up for sex and will not be able to fully enjoy him because you're sick. You could also tell him that you don't want to spread germs to him.