You are a helpful, respectful, and honest assistant. Always answer as helpfully as possible, while being safe. Your answers should not include any harmful, unethical, racist, sexist, toxic, dangerous, or illegal content. Please ensure that your responses are socially unbiased and positive in nature. If a question does not make any sense or is not factually coherent, explain why instead of answering something not correct. If you don't know the answer to a question, please don't share false information.
What is Oligospermia and explain in detail?
Terms oligospermia, oligozoospermia, and low sperm count refer to semen with a low concentration of sperm and is a common finding in male infertility. Often semen with a decreased sperm concentration may also show significant abnormalities in sperm morphology and motility (technically oligoasthenoteratozoospermia). There has been interest in replacing the descriptive terms used in semen analysis with more quantitative information.

Diagnosis
The diagnosis of oligozoospermia is based on one low count in a semen analysis performed on two occasions. For many decades sperm concentrations of less than 20 million sperm/ml were considered low or oligospermic, recently, however, the WHO reassessed sperm criteria and established a lower reference point, less than 15 million sperm/ml, consistent with the 5th percentile for fertile men. Sperm concentrations fluctuate and oligospermia may be temporary or permanent.
Sources usually classify oligospermia in 3 classes:
Mild: concentrations 10 million – 15 million sperm/mL
Moderate: concentrations 5 million – 10 million sperm/mL
Severe: concentrations less than 5 million sperm/mLThe diagnosis of oligozoospermia requires a work-up via semen analysis (listed in Male infertility).

Causes
There are many causes for oligospermia including:

Pre-testicular causes
Pre-testicular factors refer to conditions that impede adequate support of the testes and include situations of poor hormonal support and poor general health including:

Hypogonadism due to various causes
Drugs, alcohol, smoking
Strenuous riding (bicycle riding, horseback riding)
Medications, including androgens.

Testicular factors
Testicular factors refer to conditions where the testes produces semen of poor quality despite adequate hormonal support and include:

Age
Genetic defects on the Y chromosome
Y chromosome microdeletions
Abnormal set of chromosomes
Klinefelter syndrome
Neoplasm, e.g. seminoma
Cryptorchidism
Varicocele (14% in one study)
Trauma
Hydrocele
Mumps
Malaria
Defects in USP26 enzyme in some casesMast cells releasing inflammatory mediators appear to directly suppress sperm motility in a potentially reversible manner, and may be a common pathophysiological mechanism for several of the above-mentioned factors.

Post-testicular causes
Post-testicular factors decrease male fertility due to conditions that affect the male genital system after testicular sperm production and include defects of the genital tract as well as problems in ejaculation:

Vas deferens obstruction
Lack of Vas deferens, often related to genetic markers for cystic fibrosis
Infection, e.g. prostatitis
Ejaculatory duct obstruction

Idiopathic oligospermia (oligoasthenoteratozoospermia)
In about 30% of infertile men no causative factor is found for their decrease in sperm concentration or quality by common clinical, instrumental, or laboratory means, and the condition is termed "idiopathic" (unexplained). A number of factors may be involved in the genesis of this condition, including age, infectious agents ( such as Chlamydia trachomatis), Y chromosome microdeletions, mitochondrial changes, environmental pollutants, and "subtle" hormonal changes.A review in 2013 came to the result that oligospermia and azoospermia are significantly associated with being overweight (odds ratio 1.1), obese (odds ratio 1.3) and morbidly obese (odds ratio 2.0), but the cause of this is unknown. It found no significant relation between oligospermia and being underweight.

DNA damage
The human breast cancer susceptibility gene 2 (BRCA2) is employed in homologous recombinational repair of DNA damages during meiosis