Disease

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Ground squirrels are associated with the spread of Rocky Mountain spotted fever, rat bite fever, tularemia, Chagas’ disease, adiospiromycosis, and encephalomycarditis. Notably, they can serve as reservoirs for sylvatic (bubonic) plague, a highly infectious disease caused by the bacteria Yersinia pestis. Plague is transmitted to people through flea bites and direct contact with infected hosts. People and their pets can get plague if they visit or live in areas where ground squirrels or other rodents are infected. This may include rats that live in close contact with people.
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See also:
Plague in California (via CDPH)

Infected fleas can bite pets such as dogs and cats, and pets can also harbor infected fleas, bringing them in closer contact with humans.  This increases the risk of plague to people. Early diagnosis of plague in humans is vital so that it can be treated in the early stages of infection. The disease is often fatal if it remains undiagnosed. Late diagnosis may also lead to the development of lung infection (plague pneumonia, or pneumonic plague), the most serious form of the disease and the only form of plague that can be spread from person to person (by infectious droplets).
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When managing ground squirrels it is extremely important to be very vary of fleas.  When a ground squirrel dies, the fleas will jump off the carcass to find the nearest living host.  When controlling ground squirrels, it is sometimes necessary to manage fleas beforehand.  Applications of insecticidal dusts can be applied directly to burrow systems or these can be made via bait stations.  Many fumigation management options will control both ground squirrels and their fleas.  A limited number of toxic baits labeled for ground squirrel use, also contain insecticides intended to control fleas.
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See also: Flea Management  (via UC IPM)