Having one disease can make a person more likely to catch another. We say they are more susceptible, meaning they have a higher susceptibility to other diseases. This happens for several reasons.
Some diseases weaken the immune system. HIV destroys white blood cells, which are needed to fight pathogens. As the number of working white blood cells falls, the person can develop AIDS and catch infections such as tuberculosis and pneumonia far more easily than a healthy person.
Some diseases damage body tissues. Tuberculosis damages lung tissue, so other pathogens can infect the damaged area more easily.
Physical and mental health are also linked. A serious long-term physical illness can cause stress and worry, which can lead to mental illness such as depression.