During allergy season, sufferers know the drill: runny nose, watery eyes, sneezing up a storm. For the unlucky with asthma, symptoms might also include coughing and wheezing. However, asthma is not always caused by allergens like pollen and dust—non-allergenic asthma is brought on by a number of possible environmental or genetic triggers. Then there’s the common cold, which also presents with similar symptoms. So what’s causing your runny nose? In this eBook, Allergies, Asthma and the Common Cold, we review what we know about these three conditions as well as take a look at food allergies and what’s on the horizon for allergy treatment and prevention. We begin in Section 1 with what causes allergies, including “Can It Be Bad To Be Too Clean?” in which Steve Mirsky interviews Johns Hopkins School of Medicine researcher Kathleen Barnes on the science behind the hygiene hypothesis, which suggests that early exposure to bacteria and viruses leads to a stronger immune system and a reduced likelihood of developing allergies later in life. Section 2 tackles asthma, which is reaching epidemic proportions in some parts of the world. In “Why Are Asthma Rates Soaring?” Veronique Greenwood examines why the hygiene hypothesis might fall short of explaining surging asthma rates. The common cold, on the other hand, shows many of the same symptoms but is caused by a virus rather than elevated levels of IgE, as explained in Section 3 by Jordan Lite in “What’s the Difference Between Cold and Flu.” The last two sections examine treatment and prevention: Section 4 discusses interventions for food allergies specifically, including genetically modified foods and immunotherapy, whereas Section 5 looks into allergies in general. Several stories investigate how to harness our immune system, including “How Microbes Keep Us Healthy” by Katherine Harmon Courage, who describes how the equally fine-tuned “human microbiome” may be protecting us more than we know.