By default, SNMP is not enabled on hosts, either as a service or as a configured node. If you want to enable the SNMP service, in order to use this protocol, then, for each host, the proper trap destination should be configured as also a correct community.
ESXi 5.1 and later, supports SNMPv3, which provides stronger security than SNMPv1 or SNMPv2, including key authentication and encryption.
Also on vCenter Server, you can enable sending traps on different alarms, but the SNMP receivers must be set in the general configuration. If SNMP is not being used, it might be better to keep it disabled; if it is not properly configured, monitoring information can be collected from a malicious host that can then use this information to plan an attack.