Malnourished people are often more susceptible to colds and infectious diseases because they have deficiencies in their immune system.
There are two main reasons for malnourished people to be more susceptible to infections. First, malnourished people usually face unsanitary conditions where disease is quickly spread. Second, malnourished people usually have depressed immune systems because the body needs nutrition to develop proper resistance to disease.
Malnourished people do not have access to health care, sanitary conditions, and insect control. Insects and poor sanitation can spread diseases.
A malnourished person has difficulty doing normal things such as ... resisting disease. (wfp.org)
In addition, people who are malnourished do not have the vitamins and nutrition needed to develop their immune system. Even in an otherwise healthy or sanitary lifestyle, a person who does not eat properly risks infection and disease. It should also be noted that diseases often cause malnutrition.