Myelitis is inflammation of the spinal cord which can disrupt the normal responses from the brain to the rest of the body, and from the rest of the body to the brain. Inflammation in the spinal cord, can cause the myelin and axon to be damaged resulting in symptoms such as paralysis and sensory loss. Myelitis is classified to several categories depending on the area or the cause of the lesion; however, any inflammatory attack on the spinal cord is often referred to as transverse myelitis. Types of myelitis
Myelitis lesions usually occur in a narrow region but can be spread and affect many areas. Acute flaccid myelitis: a polio-like syndrome that causes muscle weakness and paralysis. Poliomyelitis: disease caused by viral infection in the gray matter with symptoms of muscle paralysis or weakness
Transverse myelitis: caused by axonal demyelination encompassing both sides of the spinal cord
Leukomyelitis: lesions in the white matter
Meningococcal myelitis (or meningomyelitis): lesions occurring in the region of meninges and the spinal cordOsteomyelitis of the vertebral bone surrounding the spinal cord (that is, vertebral osteomyelitis) is a separate condition, although some infections (for example, Staphylococcus aureus infection) can occasionally cause both at once. The similarity of the words reflects that the combining form myel(o)- has multiple (homonymous) senses referring to bone marrow or the spinal cord. Symptoms
Depending on the cause of the disease, such clinical conditions manifest different speed in progression of symptoms in a matter of hours to days.
Most myelitis manifests fast progression in muscle weakness or paralysis starting with the legs and then arms with varying degrees of severity. Sometimes the dysfunction of arms or legs cause instability of posture and difficulty in walking or any movement. Also symptoms generally include paresthesia which is a sensation of tickling, tingling, burning, pricking, or numbness of a persons skin with no apparent long-term physical effect. Adult patients often report pain in the back, extremities, or abdomen. Patients also present increased urinary urgency, bowel or bladder dysfunctions such as bladder incontinence, difficulty or inability to void, and incomplete evacuation of bowel or constipation. Others also report fever, respiratory problems and intractable vomiting. Diseases associated with myelitis
Conditions associated with myelitis include:

Acute disseminated encephalomyelitis: autoimmune demyelination of the brain causing severe neurological signs and symptoms
Multiple sclerosis: demyelination of the brain and spinal cord
Neuromyelitis optica or Devics disease: immune attack on optic nerve and spinal cord
Sjögrens syndrome: destruction of the exocrine system of the body
Systemic lupus erythematosus: a systemic autoimmune disease featuring a wide variety of neurological signs and symptoms
Sarcoidosis: chronic inflammatory cells form as nodules in multiple organs
Atopy: an immune disorder of children manifesting as eczema or other allergic conditions. It can include atopic myelitis, which causes weakness. Immune-mediated myelopathies, heterogeneous group of inflammatory spinal cord disorders including autoimmune disorders with known antibodies

Cause
Myelitis occurs due to various reasons such as infections.