1. Bacterial infection, e.g. Streptococcus agalactiae is most common and most serious infecting agent of acute attack with tendency to become chronic, Streptococcus uberis is less common, Staphylococcus pyogenes chronic, Streptococcus uberis is less common, Staphylococcus pyogenes is also common, Corynebacterium pyogenes causes a chronic mastitis complicated by abscesses difficult to treat. Other bacteria as E. coli, salmonella, proteus etc.
2. Allergic, cold, etc.
3. Injuries and wounds of udder, dirty place of living, dirty udder, faulty milking with dirty contaminated hands.
4. Large pendulous udders are more liable to injury and large teat openings are easily infected.
5. Incomplete removal of milk from the udder increases the chance of infection.
Symptoms-It may be subclinical or clinical.

Subclinical:
The symptoms are not obvious except reduction in milk yield. The abnormality is detected in test of milk by presence of white cells in high percentage but it may be a condition of inflammation due to trauma. Therefore, there should be a bacteriological test to identify the responsible organisms for better treatment accordingly. About 20 to 30 per cent of all milking cows suffer from this form.

Clinical :
The symptoms are quite obvious with reduction of milk yield more than 20 per cent, changed in colour and hard, hot, painful swollen udder.
Acute condition-In this condition one or more quarters may be affected. Later, infection may spread to the other quarters. The inflammation and induration of udder may lead to systemic disturbances. There is refusal of food, temporary suspension of rumination; rise in temperature; hard, hot, painful swelling on touch; and the colour of milk is changed. At first, a few white clots are seen in the milk. Later, the milk becomes yellowish whey containing whitish clots. Sometimes the milk becomes yellowish, thick or pus like or creamy.
In some cases, in a short time the appearance of the affected part is changed to deep purple or bluish colour and the condition becomes gangrenous by a change in colour to a greenish purple, by coldness and less sensibility on touch. The discharges also become blackish green with putrid odour and the affected area may slough in a few days.
The gangrenous condition usually occurs in dry cows and sometimes in cows just after calving. If the condition is affected to more than one quarter, the condition becomes serious with hard, painful udder. The discharge becomes foul smelling greenish yellow like pus. It is caused by corynebacterium pyogenes. It is called `summer mastitis'.

Subacute :
In this condition, there is no systemic disturbance. The appetite remains normal. There is no rise of temperature. The milk is decreased. There is difficulty in milking. The milk contains clots. The swelling and pain are gradually increased. The colour of milk is changed to creamy or yellowish grey.

Chronic :
It may be a siquil of an acute attack without any constitutional disturbance. The animal may suffer for a long time. The milk secretion is progressively decreased. The size of the affected quarter or quarters is gradually increased with localised induration. Sometimes some flakes are seen in the milk. This may be detected when the milk is compared with normal milk. The progressive destruction of secretory cells results in the complete loss of the quarter and shape and size are constricted.