Disease is a fact. Germ theory explains this fact.
That life evolves is a fact. The theory of evolution explains this fact.
They also want to argue that theories are incomplete because they are not laws. However, theories can not become laws, because the purpose of a theory is different from a law.
A theory explains a law. The law of gravity states that there is gravity- this is clearly a fact, and is concrete. Einstein's theory of gravity (or whichever theory is currently supported) explains how gravity works. Theories can be replaced if another, better explanation is formed. New theories do not need to be radically different from their predecessors, and can simply be modifications (although if the modification is very slight, it may still be considered the same theory).
So, the next time someone tries to dismiss Evolution for being "just a theory", proudly let them know you appreciate the vote of confidence.