A sulphur smell in your house should always concern you, no matter if it comes from a toilet, washing machine or any other plumbing fixture. The sulphur smell may come from sulphates that have made their way into the house's plumbing, or from sewer gasses that are leaking out of the house's vent pipes.
Hydrogen sulphide produces a sulphur or rotten egg smell. Hydrogen sulphide can be in soil where decomposing plants sit. Bacteria that breaks down the plants will also break down the sulphur inside, producing hydrogen sulphide. When it rains, the water running through the soil picks up the hydrogen sulphide deposits and transports them to the well or other source of your house's water.
Your toilet draws fresh water from the house's water supply every time you flush. When you press the toilet's handle, you let all of the water in the tank drain into the toilet's bowl, flushing the waste in the bowl down the toilet's drain pipe. Fresh water flows through the water supply valve in the wall behind or below the toilet, through the flexible water line and into the toilet through its water fill valve. The sulphur smell in the toilet will start in the tank and then transfer to the bowl as the water with hydrogen sulphide deposits makes its way through the toilet.
Sulphates in your house's water supply may act as a laxative, which may cause dehydration for some who drink the water. Hydrogen sulphides, in high concentrations, will poison people and can catch on fire. In lower concentrations, those who drink water with hydrogen sulphide in it may feel dizzy or nauseous. Once you smell sulphur in any part of your house's plumbing, abstain from drinking from the tap until you can have the water tested by professionals.
If the bacteria that breaks down sulphates has made its way into your house's plumbing, the bacteria will damage the pipes as it produces a sludge that will clog pipes and eat away at pipe walls. A sulphur smell in your toilet during rain may also indicate that your septic tank is full and needs to be emptied, since toilets sit lower in drain systems than most other drains. Ignoring the smell may lead to sewage coming into the house, which poses a health risk and can eat at the drain pipes unless cleaned out thoroughly.