If you are ill from another sickness, are you they more susceptible to typhoid?

Possible. Typhoid infection requires a large number of bacteria taken orally from fecally contaminated materials. If you have low stomach acid either from an underlying disease, or you are taking antacids it reduces the number of bacteria necessary to infect you. Also, prior use of antibiotics may open binding sites on the intestinal surface which makes it easier for salmonella typhi to attach.