The disease triangle. Click to enlarge. One of the most mystifying things that can happen in your garden is when a plant gets a disease. How did it happen? Will it spread? Will all my plants die? How can I get rid of it? The most important thing to understand about disease prevention is something called the disease triangle (drawing, right). Disease can only happen when three things coincide: you have a plant that can get sick (a host), a pathogen (like a fungus, bacterium, or virus) that can attack the plant, and environmental conditions (like humidity or drought) that promote the disease. If any one of these things is not present, the disease will not happen, so prevention involves knocking out at least one side of the triangle. Rather than waiting for a problem to pop up in your garden, consider the best defense against disease to be a good offense. What follows are 10 ways you can eliminate at least one side of the disease triangle and keep your plants healthy.