Resistance of a living organism to damage, especially through infections and poisoning, but also environmental conditions (in contrast, immunity means a complete defence mechanism or insensitivity). In viticulture, there are four main points where (depending on location and climate) resistance of grape varieties is very important: fungi, frost, phylloxera and drought (see drought and water stress). The term tolerance is often used instead in this context. Plants often develop a natural resistance to their enemies over very long periods of time up to several million years. A good example is the resistance of certain American grape varieties to phylloxera, which is a multi-stage disease, or to both types of mildew. The picture shows the difference between a non-resistant and resistant vine when a phylloxera bites the leaf. On the left, a prey-like bile with phylloxera inside and eggs laid by the phylloxera. On the right, a weak reaction without bile formation. For extremely resistant vines, there is no reaction at all when a phylloxera bites the leaf.