What is the logical operator for but? I saw a sentence like,
I am fine but he has flu.

Now I have to convert it into logical sentence using logical operators. I do not have any idea what should but be translated to. Please help me out. 
Thanks

I agree with Jiri on their interpretation. But coming from an AI background, I have a different sort of take:
Your example "I am fine but he has flu" has to do with the common knowledge between the speaker and the audience. The speaker has a certain belief of the above common knowledge. The attempt is to warn the audience that the proposition next to 'but' is unexpected, given the proposition before 'but'. 
Let us denote the proposition of a sentence $S$ before 'but' as $before(S)$ and after 'but' as $after(S)$. Lets denote the information content of a proposition $B$ when $A$ is already known as $I(B|A)$. Then, 'but' means: $I(after(S)|before(S)) > I(\lnot after(S)|before(S))$. That is, the information content (surprise) of $after(S)$ is more than $\lnot after(S)$ when $before(S)$ is already known. 
