What kind of illness did Helen have? What type of experience was it for the child?

According to the first chapter of her autobiography, Helen Keller was suddenly struck with an "acute congestion of the stomach and brain." Modern medical experts have speculated that this illness was likely to have been scarlet fever or meningitis, although it is impossible to be certain.
Helen was only in her second year of life when she became ill. Up to this point, she had been a normal, healthy baby. She could walk and talk, and interact with the world around her.
However, the experience of being sick had a dramatic impact on Helen's life, as we see from the following quote:

Then, in the dreary month of February, came the illness which closed my eyes and ears and plunged me into the unconsciousness of a new-born baby.

In other words, this illness robbed Helen of her sight and hearing, as well as the independence that she had just begun to develop.
In addition, in the next paragraph, Helen talks about the "agony" and "bewilderment" which this illness created. Helen spent all of her waking hours in considerable pain, as well as suffering from "dry" and "hot" eyes. Despite her best efforts, Helen's mother was unable to soothe her daughter's pain or save her sight and hearing.
Fortunately, this illness disappeared as quickly as it came. Helen, however, would never regain her sight or hearing.