a tendency to be excessively or unreasonably distrustful or suspicious of people;
e.g. Her shyness is due to her paranoia that people are always judging her.
(noun)
a mental illness that involves the delusion that one is being targeted for persecution by everyone, often in combination with baseless jealously and/or an exaggerated sense of self-importance;
e.g. He is affected with paranoia and is in constant fear for his safety.