Humidifiers are required to make up for moisture lost to the outdoors in winter. A house built to today's standards using today's storm windows sometimes have a problem getting rid of moisture. So a humidifier is required in a poorly-built house that has a warm air system that forces air out of gaps in ductwork in attics and crawl spaces. A way to prevent disease from humidifiers is to clean them often and use a biocide in the water pan to prevent growth.
A better way to eliminate humidifiers is to live in a newly built house with hot water heat so you don't need humidification at all.