Fungus MUST be removed before you start painting or it is very likely that it will grow and ruin the hours of painting that you've done, only to leave a more difficult problem to solve later on. The fungus may not be apparent at first sight, so it is important to carefully inspect surfaces to ensure that there is none there. It is commonly found in bathrooms, kitchens, and utilitary rooms so make sure that you check these areas well.
To remove fungus, treat the area with HG Mould Spray generously, applying it to all surrounding areas too. After 30 minutes of letting the wash get to work, clean the area with with a mild detergent, and then rinse with water.
Regardless of whether fungus is present or not, you should ensure that the walls are clean and degreased before you paint them, so wash the surfaces with mild detergent or sugar soap and rinse thouroughly.