There are three groups of factors which are harmful to the health of the urban poor. The first includes the direct effects of poverty i.e. low income, limited education and insufficient diet. The second factor relates to the man-made conditions of the living environment including poor housing, overcrowding, pollution and an increased exposure to infectious diseases. The third factor involves social and psychological problems encompassing instability and insecurity. The excessive vulnerability of the urban poor and their exposure to pathogenic agents means that infectious diseases and malnutrition are severe health problems in slums. The socio-economically disadvantaged in general tend to be more vulnerable, both physically and economically, and this needs to be considered when interpreting environmental health differentials. With their greater levels of exposure to poor sanitation, overcrowded living conditions, inadequate nutrition, social stresses, exposure to environmental pollutants and limited access to health care, they are more likely both to get sick and suffer prolonged periods of illness.