Ascaris is transmitted via contaminated soil. The soil gets contaminated by Ascaris eggs, which need to be in the soil for a while to become infective. There is no direct person to person transmission of Ascaris infection. Those with a weak immune system are much more susceptible to Ascaris infection. It is almost impossible to completely avoid exposure to infective ascaris eggs, as they may be on produce,and everything from door knobs to many other everyday things(apparantly these microscopic eggs are sticky and stick to things), so wash your hands well after handling raw produce, before touching other foods or other items(I guess one can get paranoid about this, worry that they will handle some raw produce, then get some parasite eggs on the refrigerator door handle). The infective eggs may even be in some tap water. Washing vegetables very well before eating them, washing hands well before eating anything, and washing any food that fell on the floor very well before eating it will limit your exposure to the eggs. Taking 10,000 IU a day(for an adult) of retinol(fully formed vitamin A) greatly reduces the chance of reinfection. Eating ground cloves every day will also help reduce the chance of reinfection. Having candida overgrowth makes one much more susceptible to ascaris infection. I think when I had my ascaris infection a few years ago, I had a vitamin A deficiency. I was surprised to see the studies from Africa where ascaris infection is rampent about how effective retinol(fully formed vitamin A) is in preventing reinfection with ascaris.