How did mr.frisby die?

Mr. Frisby, the father of the protagonist in "Mrs. Frisby and the Rats of NIMH" by Robert C. O'Brien, died due to a combination of illness and the effects of the experiments conducted on him as a lab rat at NIMH. His weakened state ultimately led to his demise, leaving Mrs. Frisby to care for their family alone. His death serves as a catalyst for the events that unfold in the story, driving Mrs. Frisby to seek help for her sick son.