What causes tooth decay? Is it acid from food particles or an infection?

The research money is on a bacterium - to be specific, Streptococcus mutans - that thrives in dental plaque. It can live in acid or alkaline conditions, and even if you assiduously brush all the food residue away every day, if you have tartar on your teeth it will survive and eat away at them. The good news is that Californian doctor Wenyuan Shi claims to have developed an antibacterial agent that targets only S. mutans and not the non-nasty microbes in our mouths. It seems that taking it for a day, then at fairly long intervals thereafter, may arrest and prevent decay. It's called novispirin G10, but don't hold your breath waiting for it - it will take several years before reaching the market, if ever.

Small black spots appear on my mouse pad every few weeks. I have to clean them off with soapy water. Could they be to do with the fact that I sneeze most mornings when I go into my office?

It's possible. It sounds as if you are growing colonies of microscopic fungal spores on the pad. Fungal spores in the air could cause you to sneeze. The fact they return after you clean the pad suggests there's a source of fungi in your room. Is it properly ventilated? Are the walls damp? Has it been a while since you spring-cleaned, including carpets and furniture? Have you got loads of files and books in the room that are gathering dust? They all need checking and, where possible, cleaned or ditched.

#183; Do you have a question for Dr Smith? Email doctordoctor@theguardian.com