Conversation about the cup which I only used just one time

At the moment when I tried to go　back home.

I notice that made of prastic cup, which temporary use to drink some had leave on the my work desk.
and then cover it tissue paper

The designer who seat next to me　say 'Why not throw it away?, Why are you covering the cup with tissue?'

I said 'mottainai' because this cup can drink yet. and not need take other cup.

She continued her opinion.

That method is filthy. You will get sick?

However, she had been lying with influenza recently.

I am generally not sure whether to use the once-used cup again, or to throw away it and use a new one.