Otherwise, it's usually the result of a mild infection.

A mild infection may not have any symptoms.

Enlarged lymph nodes may be caused by a number of conditions, ranging from very mild infections to serious malignancies.

This was not known until the virus was isolated in cultured cells and many people were shown to have had mild infections that did not lead to poliomyelitis.

Younger infants with higher residual levels of antibodies would experience milder infections and these would be more likely to go unrecognized or be dismissed as teething difficulty.