He seems to have a flu.

GPY

Moonrise

GPY

Sorry, I forgot to mention that we can also say "the flu", so e.g. "She has flu" and "She has the flu" are both OK.

Is there a difference in meaning between them? I mean when saying "the flu", does it necessarily refer to a certain type of flu?

No. There is no difference in meaning. The use of "the" in "the flu" is somewhat unpredictable, as is the use of articles with illnesses/diseases generally (albeit most take no article, either definite or indefinite). For example, we say that someone has "flu" or "the flu", never "a flu", but that they have "a cold", never "the cold" or "cold", and yet again we can say that someone has "toothache" or "a toothache", but never "the toothache".

Thanks a lot for clearing it up.