Infectious diseases are caused by bacteria, viruses, fungi, protozoans, prions and parasites (worms). The pathogens aren’t visible to the naked eye, with the exception of some types of worms. Zoonoses can be transmitted through direct contact, through indirect contact and by vectors. Direct contact is obvious: this is when a human touches, pets or holds an animal, or when an animal bites or scratches. Eating infected food (milk, meat or eggs) is also considered direct contact. Indirect contact isn’t as obvious. Sometimes pathogens excreted by the animal (e.g. in excrement) can survive for a long time in the soil. A human can then be infected through contact with the soil. Transmission of infection by vector is another form of indirect contact. Some pathogens are dependent on vectors for transmission: another animal (mosquito, fly, tick) provides transportation. Infection can occur when a human is stung or bitten by an infected vector.
Because the pathogens aren’t visible to the naked eye and animals are often not visibly sick (even though carrying disease), it isn’t always obvious how transmission occurs. Furthermore, infection through indirect contact is puzzling: the connection to the animal – that may or may not be sick – is difficult to make. Many people know that they must be vigilant after being bitten by an animal, but the link between working in the garden and showing symptoms of disease is often missed.