Hands carry a lot of harmful microbes through contacts with germ-infested surfaces such as door knobs, dust covered surfaces, nose blowing, sneezing, handling dirty and rusty stuff, and even through handshakes.
Handwashing with soap or sanitizers before eating is necessary to prevent hand to mouth transferred diseases and gastrointestinal infections.
Viral and bacterial infections transferred through hand to mouth and physical contact are -dysentery, diarrhoea, flu, fever etc.