For an infection to occur, how many conditions must be present? Input the correct answer in numeric form into the space provided.

To establish an infection, there are typically five conditions that must be present. These conditions are often summarized using the mnemonic "Chain of Infection," which includes:

1. Pathogen: A disease-causing microorganism such as bacteria, viruses, fungi, or parasites.
2. Reservoir: The natural habitat where the pathogen lives and multiplies, which can include humans, animals, or the environment.
3. Portal of Exit: The route by which the pathogen leaves the reservoir, such as through respiratory secretions, blood, or bodily fluids.
4. Mode of Transmission: The mechanism by which the pathogen is transmitted from the reservoir to a susceptible host, which can be direct (e.g., person-to-person contact) or indirect (e.g., contaminated surfaces).
5. Portal of Entry: The way for the pathogen to enter a susceptible host, which can be through mucous membranes, broken skin, or respiratory tract.

Therefore, for an infection to occur, the correct answer is 5.