A nucleus consists of a nuclear membrane that surrounds it, within the membrane there is a nucleolus which can be compared metaphorically to the yolk of an egg in appearance. The nucleolus is surrounded by nucleoplasm which is quite like the cytoplasm.
        
                    
             
        
        
        
The flu vaccine changes every year. The flu vaccine is based on a predicted strain that is anticipated to be the most infectious that year. Thats why people who sometimes get the vaccination still get the flu, they don't have the antibodies for the strain they were infected with.  Strains for viral disease don't change very much and usually the vaccination for them cover them well.