An in vitro method for diagnosing or predicting whether a human patient, who is a Pneumocystis jirovecii carrier has or develops Pneumocystis pneumonia, wherein said method comprises i. in the RNA material of a sample of biological fluid previously obtained from the respiratory tract of said human patient, quantifying the RNA transcripts of two different P. jirovecii mitochondrial genes to obtain a value of quantification of the RNA transcripts of a first P. jirovecii mitochondrial gene and a value of quantification of the RNA transcripts of a second P. jirovecii mitochondrial gene, wherein at least one of said two different P. jirovecii mitochondrial genes is a P. jirovecii gene, the sequence of which transcribes into a P. jirovecii ribosomal RNA, wherein said first P. jirovecii mitochondrial gene is the P. jirovecii gene, the sequence of which codes for the Cytb protein or is a P. jirovecii gene, the sequence of which transcribes into a P. jirovecii ribosomal RNA, and wherein said second P. jirovecii mitochondrial gene is a P. jirovecii gene, the sequence of which transcribes into a P. jirovecii ribosomal RNA, ii. calculating the ratio of the value of quantification of the RNA transcripts of said first P. jirovecii mitochondrial gene of i. to the value of quantification of the RNA transcripts of said second P. jirovecii mitochondrial gene of i., iii. comparing the value of the ratio of ii. to a threshold value, wherein, when the value of the ratio of ii. is equal to or lower than said threshold value, said human patient is diagnosed or predicted to be at high risk of having or developing PCP, wherein, when the value of the ratio of ii. is higher than said threshold value, said human patient is diagnosed or predicted to be at low risk of having or developing PCP, wherein said threshold value has been predetermined by comparing the values, or the distribution of the values, that the ratio of the value of quantification of the RNA transcripts of said first P. jirovecii mitochondrial gene to the value of quantification of the RNA transcripts of said second P. jirovecii mitochondrial gene takes in reference human cohorts of P. jirovecii carriers, who have been pre-established as a function of their status of P. jirovecii carriers, who have or develop PCP, or of P. jirovecii carriers, who do not have and do not develop PCP, in order to classify said human patient into that of those reference cohorts to which it has the highest probability of belonging, and wherein the quantification of said step i. is performed by (cDNA) reverse-transcription and PCR amplification, in particular real-time PCR. i. in the RNA material of a sample of biological fluid previously obtained from the respiratory tract of said human patient, quantifying the RNA transcripts of two different P. jirovecii mitochondrial genes to obtain a value of quantification of the RNA transcripts of a first P. jirovecii mitochondrial gene and a value of quantification of the RNA transcripts of a second P. jirovecii mitochondrial gene, wherein at least one of said two different P. jirovecii mitochondrial genes is a P. jirovecii gene, the sequence of which transcribes into a P. jirovecii ribosomal RNA, wherein said first P. jirovecii mitochondrial gene is the P. jirovecii gene, the sequence of which codes for the Cytb protein or is a P. jirovecii gene, the sequence of which transcribes into a P. jirovecii ribosomal RNA, and wherein said second P. jirovecii mitochondrial gene is a P. jirovecii gene, the sequence of which transcribes into a P. jirovecii ribosomal RNA, ii. calculating the ratio of the value of quantification of the RNA transcripts of said first P. jirovecii mitochondrial gene of i. to the value of quantification of the RNA transcripts of said second P. jirovecii mitochondrial gene of i., iii. comparing the value of the ratio of ii. to a threshold value,