Blood phase syndrome is caused by invasion of pathogenic febrile factors into yin blood and leading to disturbance of blood, generation of wind and consumption of yin. This syndrome appears at the critical stage of epidemic febrile disease.

Clinical manifestations:
Worsened fever in the night, restlessness, or even delirium, mania, appearance of purplish or blackish macules and eruptions, or hematemesis, epistaxis, hematochezia, hematuria, deep-red tongue, and rapid pulse; or convulsion, stiffness of neck, episthotonos, upward staring of eyes, lockjaw and taut and? rapid pulse; or continuous low fever, evening fever with alleviation in the morning, feverish sensation over the five centers (palms, soles and chest), emaciation and dispiritedness, deafness and thin pulse; or tremor of hands and feet and flaccidity.

Analysis of the symptoms:
Worsened fever in the night is due to exuberant heat in blood phase scorching blood; restlessness, delirium and mania are due to heat disturbing the mind; appearance of purplish or blackish macules and eruptions and various bleeding are caused by heat scorching collaterals and causing extravasation of blood; deep-red tongue, and rapid pulse are signs of exuberance of heat in blood phase; convulsion, stiffness of neck and episthotonos are due to exuberant heat in blood phase to scorch vessels and tendons; low fever, evening fever with alleviation in the morning, feverish sensation over the five centers are due to retention of pathogenic heat consuming liver and kidney yin and internal disturbance of asthenia-heat due to yin asthenia; emaciation and dispiritedness and thin pulse are due to deficiency of yin essence and malnutrition of the body; dispiritedness is due to malnutrition of spirit; deafness is due to consumption of kidney yin and malnutrition of ears; tremor of hands and feet and flaccidity are signs of endogenous asthenia-wind due to consumption of liver yin and malnutrition of tendons.
    
Exogenous epidemic febrile disease usually starts from defensive phase, gradually developing into qi phase, nutrient phase and blood phase as the pathological conditions are gradually getting worsened. Such a progress is called due transmission. If pathogenic factors enter defensive phase and directly penetrates into blood phase without passing through qi phase, coma and delirium will be caused. Such a progress is known as adverse transmission. Besides, there are still some other ways of transmission, such as "simultaneous involvement of defensive phase and qi phase", "heat in both qi phase and nutrient phase" and "heat in both qi phase and blood phase".