Yoghurt is fermented milk. Yoghurt has traditionally been eaten or drunk in many cultures from India through to Western Europe and is now popular whereever there is a western culture as well.
Soy yoghurt is a fermented product made from Soya beans and water that's similar to yoghurt. Unless you read the label you probably won't tell from the taste which is which. Soy yoghurt is also healthy provided it's unsweetened.
You need to take care over hygiene. Here is what a website that understands food hygiene recommends. "Store leftovers in the fridge within two hours of cooking and eat them within three days." Also foods like milk, yoghurt and tinned food should be eaten within three days of opening the container. Plan how to use a large carton of yoghurt within three days before you open it. If you end up with yoghurt that's been open too long put it into a dish early during cooking and boil it thoroughly.
For many/most people cheese is likely a more convenient staple than yoghurt as it can be stored safely in the fridge for longer.