Pneumonia is more common in 3-month-old babies, and common types include Chlamydia trachomatis pneumonia, respiratory syncytial virus pneumonia, Streptococcus pneumonia, Staphylococcus aureus pneumonia, etc. 1. Chlamydia trachomatis pneumonia: caused by genital infection of the mother, no fever or low fever with severe cough. 2. Respiratory syncytial virus pneumonia: 2-7 months after birth is the peak age of onset, characterized by wheezing and profuse runny nose. 3. Streptococcus pneumonia: the most common cause of bacterial pneumonia, manifested by chills and fever, coughing rust-colored sputum. 4. Staphylococcus aureus pneumonia: acute onset, severe and rapid development. Systemic moderate symptoms are obvious, and may be complicated by pus chest. If you find that your 3-month-old baby has the above symptoms, you should go to the hospital in time to clarify the cause of the disease and get treatment under the guidance of the doctor.