What Causes Weakness, Joint Pain And Fever?

My dad who is 69 years old had a bout of flu recently and since then has weakness which like he never had before and now he's suffering from severe joint pain. I have never seen his so weak before and the joint pain is causing him difficulties in sitting and getting up.
He had a H1N1 vaccination done about a week before he went down with the flu. So is tehre any relation between the vaccinatio nand his current condition? In nay case what shold be the remedy? He has been seeing various doctors since he went down with the flu, but to no avail. Thanks.