It's a sensitive topic when it comes to bad breath. But halitosis is a general problem and can happen to the all of us. Eating certain food items like garlic or fish can only make the smell worse. 
Bad breath or halitosis is a foul-smelling odour that expelled from mouth cavity. Furthermore, family members, friends, and colleagues may not feel comfortable in telling this to the concerned person.
One of the best ways to know about this problem is by checking it yourself and treat it.

WHAT ARE THE MOST COMMON CAUSES OF BAD BREATH?
Anyone can get bad breath, even people who are careful about their oral health can all suffer from this.
It has less to do with oral hygiene and more to do with things like food you eat, certain medications and lifestyle habits that favour to anaerobic, sulfur-producing, bad breath related bacteria.
Food: Eating certain foods, such as onions, garlic, and spices, also can cause bad breath.

Tobacco products: Smokers and oral tobacco users are also more likely to have gum disease, another source of bad breath.

Poor dental hygiene: If you don't brush and floss daily, food particles remain in your mouth, causing bad breath.

Dry mouth: Saliva helps cleanse your mouth, removing particles that cause bad odours. A condition called dry mouth or xerostomia can contribute to bad breath because the production of saliva is decreased.

Medications: Some medications can indirectly produce bad breath by contributing to dry mouth. 

Infections in your mouth: Bad breath can be caused by surgical wounds after oral surgery, such as tooth removal, or as a result of tooth decay, gum disease or mouth sores.

Other mouths, nose and throat conditions: Bad breath can occasionally stem from small stones that form in the tonsils and are covered with bacteria that produce odour.

Other causes: Diseases, such as some cancers, and conditions such as metabolic disorders, can cause a distinctive breath odour as a result of chemicals they produce.