The exact causes of cancer of the stomach aren’t fully understood. It's thought that most adenocarcinoma stomach cancers develop as a result of a combination of risk factors. Factors that can affect the risk of stomach cancer include:

• Gender – it is more common in men than in women.
• Age – the risk of developing stomach cancer increases as we get older. More than 9 out of every 10 people who develop stomach cancer are over 55 years old.
• Helicobacter pylori (H pylori) infection – having H pylori infection in the stomach over a long period of time can increase the risk of stomach cancer.
• Diet – eating a diet that is high in salt, pickled foods and processed meats such as sausages and bacon can increase the risk. Over the past 30 years the number of people who develop stomach cancer in the UK has fallen. This is probably related to changes in our diet, particularly the use of refrigeration, which has led to us eating more fresh food and less smoked and pickled food.
• Smoking tobacco – the longer a person smokes for and the more cigarettes they smoke, the greater the risk.
• Medical conditions – such as long-term acid-reflux and Barrett’s oesophagus can increase the risk. In these conditions acid from the stomach affects the gullet or the area where the gullet joins the stomach (the gastro-oesophageal junction). 
•  Conditions which cause lower than normal levels of acid in the stomach – such as atrophic gastritis or some types of stomach surgery, can increase the risk.
• Pernicious anaemia – a type of anaemia that is linked to changes in the lining of the stomach, can slightly increase your risk.
• Family History – having a brother, sister or parent with stomach cancer can increase your risk. It's thought that this is because close family members are likely to share some risk factors for stomach cancer in common, such as eating a similar diet or having H pylori infection. But shared genes may play a small part.
• Genes – in a very small number of families, genes may play a greater part in increasing cancer risk. This is more likely when two or more people on one side of the family are affected, especially if the cancer develops at a younger age (less than 50 years). In these families there may be a faulty gene that increases the risk of getting stomach cancer.

Stomach cancer is not infectious and can't be passed from one person to another.