The array of foods we need is wide as are the nutrients we require to maintain a healthy body. A deficiency in any nutrients can result in your body becoming weak or getting sick. Our bodies and their make up are made of many chemicals and if we don't get replacements for these chemicals we will end up sick.
We get these nutrients through food, drink and if we are short of them through supplements like vitamin tablets.
For example all our bodies rely on protein for muscles, which is found in meat and other foods. If we don't have enough of this we become weak and will suffer and array of problems.
One of the most famous deficiencies was among sailors who travelled the seas during the middle ages. These sailors were lacking in vitamin C, which is used to ensure we have a healthy immune system. These sailors suffered a disease called 'Scurvy' which caused them to be weak and useless for the physical tasks required on a ship. To combat this, sailors were given large amounts of limes when aboard ships. This meant they got enough vitamin C and weren't ill.
This is a severe example of a lack of a balanced diet. People nowadays in the first world who don't have a balanced diet may suffer less drastic short term consequences but people may suffer fatigue or other issues that will mean they are not at their peak.
Get a balanced diet by eating a diet full of all foods including fruits and vegetables.