Having one disease can make a person more susceptible to other diseases. This means they are more likely to catch or develop them. It happens in a few different ways.
First, a disease can weaken the immune system. HIV destroys white blood cells, so the body cannot fight off other infections. A person with HIV can catch tuberculosis easily because their weakened immune system cannot destroy the bacteria.
Second, a disease can damage the physical barriers that normally protect the body. A cold virus can damage the cilia that sweep mucus out of the airways. Bacteria can then reach the lungs more easily and cause a second infection.
Third, a disease can cause malnutrition. Repeated diarrhoea, for example, stops a person absorbing nutrients. A poor diet weakens the immune system, so the person becomes more susceptible to other infections.