No, no flu.
Death in the brooder is due to a few things....either your temp is not correct and they are over heating, becoming chilled, there is not enough oxygen circulation, not enough room, they are killing each other, starving to death or dehydrating and if you hatched these yourself they may have some bacterial infection from a dirty incubator.
So....you want a brooder that has 4 solid walls and a solid floor. Do not cover the lid of the brooder with anything but hardware cloth wire for good heat and oxygen exchange. The brooder needs to have enough room to give each bird a 1/2 square foot of space, no less. As they grow you will need to accommodate them. Use a good thermometer you know that is accurate and place it on the floor directly under the heat source. Do not brood them outside in the winter nor in a place that has drafts or fluctuating temps.
Use a red or infra red bulb for heating. Keep the heat on one side of the brooder and the food and water off to the other so this way they have to leave the heat to get to the goods. This gives them cool spots in the brooder which they do need.
Crush the food and sprinkle on paper towels all around the feeder area for the first week. Tap with your finger at the feed like you are eating it too. Dip everybody's beaks in the water a few times so they know where it is located.
Lower your heat by 5 degrees each week for 5 to 6 weeks.
Keep the brooder very clean and dry. Chicks 2 to 6 weeks of age are susceptible to Coccidiosis and this comes from coming into contact with too much poop and wet litter. So keep it clean.
