Critical Windows in the Development of Asthma Endotypes and Phenotypes in High-Risk Toddlers

Inclusion Criteria: * Age 18 months to 36 months at first research encounter * For enrollment in Emergency Department (ED)/Inpatient Visit: Presentation to the ED/Inpatient with an acute wheezing episode AND either evidence of at least one prior wheezing episode documented in the electronic medical record (i.e., wheezing, pneumonitis, bronchiolitis, reactive airways disease, or asthma) that occurred ≥ 14 days from current episode, or caregiver report of one of the following: * Child has had wheezing or whistling in the chest at any time in the past lasting at least 24 hours and occurring at least 14 days prior to the current ED visit * Child has been treated with albuterol or used an inhaler or a nebulizer at least 14 days prior to the current ED visit * Caregiver has been told that their child has bronchiolitis, asthma, or reactive airways disease at least 14 days prior to the current ED visit * For enrollment in Visit 1: At least three separate wheezing episodes, separated by at least 14 consecutive days without a wheezing episode (wheezing lasting ≥ 24 hours), as reported by parent/caregiver or patient's care provider and/or documented in the patient's medical record * Third episode may be at time of presentation to ED or admission as inpatient (IP) to hospital when recruitment would occur (Visit ED/IP) * Bronchiolitis and wheezing/asthma exacerbations are considered wheezing episodes * The wheezing episode does not require treatment with bronchodilators * Live within a US Office of Management and Budget-defined Metropolitan Statistical Area (MSA) and meet at least one of these criteria: 1. Have publicly funded health insurance AND/OR 2. Live in a census tract within an MSA where ≥10% of families have income below the poverty level Exclusion Criteria: * Wheeze attributed to alternative diagnoses other than bronchiolitis or asthma, including congenital anatomical abnormalities, foreign body aspiration, chronic lung disease of prematurity, other chronic lung disorders such as cystic fibrosis, and cardiac, immune, and gastrointestinal disorders. * Gestational age \< 32 weeks.