Bronchiolitis: This is the most likely diagnosis given the age of the patient and the symptoms described, such as thick whitish nasal secretions and a flushed face after crying. Bronchiolitis is a common lower respiratory tract infection in infants, typically caused by the respiratory syncytial virus (RSV), and is characterized by inflammation and congestion of the small airways.

Viral Pneumonia: Although pneumonia is a consideration, viral pneumonia could present similarly to bronchiolitis, especially in the absence of fever. The symptoms of cough, nasal congestion, and the response to nebulization could be consistent with viral pneumonia.
Upper Respiratory Tract Infection (URTI): Given the symptoms of nasal congestion with thick whitish secretions, an upper respiratory tract infection could be a likely diagnosis. However, the presence of significant respiratory distress or response to nebulization might lean more towards a lower respiratory tract infection.

Bacterial Pneumonia: Although less likely given the absence of fever, bacterial pneumonia is a critical diagnosis not to miss due to its potential severity and need for antibiotic treatment. Symptoms can sometimes be subtle in young infants.
Congenital Heart Disease: While not directly related to the respiratory symptoms, the presence of significant respiratory distress or failure to thrive in an infant warrants consideration of underlying congenital heart disease, which could exacerbate respiratory infections.
Pertussis (Whooping Cough): Especially in a young infant who may not have completed the primary vaccination series, pertussis is a critical diagnosis to consider due to its potential for severe illness and the need for isolation and treatment.

Cystic Fibrosis: Although rare, cystic fibrosis could present with recurrent or severe respiratory infections early in life. The presence of thick, sticky secretions might raise suspicion, but this would be an uncommon diagnosis in the context provided.
Primary Ciliary Dyskinesia: A rare genetic disorder that affects the motor function of cilia, leading to recurrent respiratory infections. This diagnosis might be considered if there are recurrent infections or if the infant fails to thrive, but it is much less common than other diagnoses listed.