Any disease-causing organism is referred to as a pathogen. Example- Viruses, bacteria, etc.
Pathogens are capable of causing harmful diseases to organisms and therefore are said to be harmful.
In gram-negative bacteria, the peptidoglycan layer is much thinner than that of gram-positive bacilli.
Gram-negative bacteria are therefore harder to kill because of their harder cell walls.
When their cell wall gets disturbed, the gram-negative bacteria start releasing endotoxins which could cause a problem in an already persistent disease making it worsen the condition.