What classification applies to any substance that can cause acute or chronic illness in the human body?

The classification that applies to any substance capable of causing acute or chronic illness in the human body is toxic. Toxic substances can result in various harmful effects, depending on factors such as the dose, exposure duration, and the individual's sensitivity. The term encompasses a range of agents, including chemicals, plants, and even certain biological materials that can lead to health issues upon exposure or ingestion.

In contrast, corrosive substances can cause damage to living tissue or severe corrosion of materials but do not necessarily impart toxicity in the way that is being outlined in the question. Flammable materials can ignite and pose safety hazards but are not defined by their ability to cause illness. Non-toxic substances, as the name implies, are considered safe for use and do not produce toxic effects on the human body. This distinction makes toxic the correct classification for substances that can lead to illness.