The purpose of the study is to determine if the clinical course of pneumonia is more severe when both, bacterial and viral pathogens are find as possible causative agent and how does it affect treatment.

Basic demographic data of the patients (age, gender), data on concomitant diseases and epidemiological circumstances will be collected

Severity of illness will be assessed by APACHE II (at day 1) and SOFA (at day 1,2,3 and 7) scoring system

Following laboratory tests will be performed at day 1, 2,3 and 7.:

• blood count analysis
• differential blood count
• C- reactive protein, procalcitonin
• glucose, urea, potassium, sodium, chloride, magnesium, creatinine, bilirubin, protein, albumin, alkaline phosphatase, aspartate aminotransferase, alanine transaminase, gamma-glutamyl transpeptidase , alpha-amylase, lipase, creatine kinase, lactate dehydrogenase, myoglobin
• arterial blood gas analysis
• tests of hemostasis.

Additional 2 mL of blood will be taken at day 1 and day 21 (or later), due to paired serologic testing

To establish the presence of potential pathogens we will perform following microbiological investigations:

• cultivation of 2 pairs of blood cultures in all patients
• cultivation of sputum or quantitative cultivation of tracheal aspirate in nonintubated patients
• quantitative cultivation of tracheal aspirate or bronchoalveolar lavage in intubated patients
• testing the presence of soluble Legionella antigen in urine in all patients
• testing the presence of genetic material of respiratory viruses (influenza A, influenza B, respiratory syncytial virus , adenovirus, bocavirus, coronavirus, metapneumovirus, parainfluenza virus, rhinovirus) by qualitative polymerase chain reaction (PCR) in nasopharyngeal swabs in all patients, in the tracheal aspirate in nonintubated patients and in tracheal aspirate or bronchoalveolar lavage fluid in intubated patients
• testing the presence of mycoplasmal, legionella and chlamydial DNA by qualitative PCR in nasopharyngeal smears in all patients, in the tracheal aspirate in nonintubated patients and in bronchoalveolar lavage fluid or in tracheal aspirate in intubated patients

Inclusion Criteria: diagnosis of

• severe community acquired pneumonia or
• severe hospital acquired pneumonia or
• ventilator--associated pneumonia

Exclusion Criteria:

• antibiotic treatment of actual episode of pneumonia for more than 24 hours

Adult patients admited to Intensive Care Unit Department of Infectious Diseases University Medical Centre Ljubljana due to severe pneumonia or pneumonia will develop during their stay in ICU.