Endocarditis is caused by bacteria in the bloodstream multiplying and spreading across the inner lining of your heart (endocardium). The endocardium becomes inflamed, causing damage to your heart valves.

Your heart is usually well protected against infection so bacteria can pass harmlessly by.

But if your heart valves are damaged or you have an artificial valve, it's easier for bacteria to take root and bypass your normal immune response to infection.

Small clumps of bacteria can develop at the site of the infection. There's a risk of these clumps acting in a similar way to blood clots, travelling away from the heart and blocking the blood supply to the organs. This can cause organ failure or trigger a stroke.

There are several ways that bacteria can enter your blood.

Mouth

Everyday activities, such as brushing your teeth or chewing your food, can sometimes allow bacteria to enter the bloodstream.

The risk is increased if your teeth and gums are in bad condition because it makes it easier for bacteria to enter.

Infection

Bacteria can spread from the site of a pre-existing infection, such as a skin infection or gum disease.

Needles and tubes

Any medical procedure that involves placing a medical instrument inside the body carries a small associated risk of introducing bacteria into your bloodstream.

There are a number of things that can make your heart more vulnerable to infection and increase your chance of developing endocarditis.

These include:

• heart valve disease
• having prosthetic valves
• hypertrophic cardiomyopathy
• injecting drugs
• congenital heart disease
• you've had endocarditis before

Heart valve disease

Heart valve disease is a general term describing health problems that damage the valves of the heart.

Two types of heart valve disease known to increase your risk of endocarditis are:

• valvular stenosis – where the valve(s) of the heart become narrowed, disrupting the blood flow through the heart
• valvular regurgitation – where the valve(s) of the heart do not close properly, causing blood to leak back in the wrong direction

Heart valve disease can be either: 

• congenital heart disease – where you're born with the condition
• acquired – where you develop the condition in later life

Causes of acquired heart valve disease include:

• a previous heart attack – a heart attack can damage the muscles that surround and support the valves, preventing the valves from functioning properly
• high blood pressure (hypertension) – without treatment, high blood pressure can weaken the tissue around the valves
• rheumatic fever – a type of bacterial infection that can damage the heart

Rheumatic fever is rare since the introduction of antibiotics. But older people who had rheumatic fever during childhood may go on to develop heart valve disease.

Prosthetic valves

The prosthetic (artificial) valves used in aortic valve replacement surgery replace heart valves that have been damaged by heart valve disease.

But bacteria can also take root around prosthetic valves, which can occasionally trigger endocarditis.

Hypertrophic cardiomyopathy

In hypertrophic cardiomyopathy, the heart muscle cells enlarge and the walls of the heart chambers thicken.

The chambers are reduced in size so they cannot hold much blood, and the walls cannot relax properly and may stiffen.

Injecting drugs

People who inject illegal drugs such as heroin or methamphetamine (crystal meth) have an increased risk of developing endocarditis.

This is because unsterilised needles allow bacteria to enter the bloodstream and repeated injections make the skin more vulnerable to infection.

Fungal endocarditis

Endocarditis caused by a fungal infection is rarer than bacterial endocarditis, and usually more serious.

You're more at risk of fungal endocarditis if you:

• inject drugs
• have a history of heart surgery
• have a central venous catheter – a tube connected to a vein in the neck, groin or chest, which is used to deliver medicines or fluids to people who are seriously ill
• have a weakened immune system – either as a result of a health condition affecting the immune system, such as HIV, or as a side effect of certain types of treatments, such as chemotherapy.