Virus do not go on vacation in the summer. The problem is your immune system tends to get weaker in the winter due to less sunlight and the angle of the sun. You become Vitamin D deficient. Vit. D3 is made on the skin from sunlight. It is technically a horomone, and different from all the vitamins. Proper blood levels strengthen the immune system and prevent cytokine storms. This is when your own immune system over-reacts and damages your own tissue. It is thought that the rapid deaths of young people due to flu after WW1 were actually due to cytokine storms. The storm damages and inflames lung tissue, and you die from excessive blood and fluid in the lungs, from suffocation. Young people generally have stronger immune systems so the storm tends to be more pronounced. In addition, you had many young people coming home from terrible conditions during trench warfare, which could have been an additional factor. Twenty minutes of exposure half clothed in the mid-day summer sun produces about 10,000 units of D3. The body can store this for awhile. Sunscreen however blocks production. It is best to take supplements during the winter months. This will keep your system strong and well regulated. As far as the needle goes, you do not really know what you will be getting.