The verb ail means to be sick or unwell — or cause to be so. If you feel a general malaise, someone may ask what ails you, though you may just need a vacation from work or school.
The verb ail is used for things that are metaphorically unwell or unhealthy. Politicians, for example, will often tell their constituents that the only cure for what ails the country is to vote for them. Ail is used when referring to non-specific illnesses. So you are sick with the flu, or troubled by allergies, but you are not ailing because you know what's wrong. It's when you're sick but it hasn't been diagnosed that you are ailing.