The Souls have efficient technology and medicine (much of which was created for them by the Spiders ) - essential to the colonization of planets and maintenance of those colonies. (Note some of the below information is unclear
It is vaguely implied that originally the Souls either relied on hijacked Vulture technology or on those they created themselves for the colonization process. Afterward they might have made use of whatever tecnology the races whose bodies they assimilated used. Once they found the Spiders, a race without much emotion, but with high intelligence in certain other fields, their tecnology received a boost. When assimilating a new planet the Souls abduct a few examples of the race they target and perform tests to develop medicines for use on the hosts.
The main users of weapon tecnology among souls are Seekers on certain planets. The main users of medicinal application among Souls are the Healers and their assistants.
When the Souls assimilated Earth they replaced the medical system with Healers and Comforters and the aeronautics system with a system af Shuttles.
Healer is a special Calling (some others being Comforter and Seeker). A Healers job is to repair damage to especially host bodies, but also possibly the Souls themselves (though this is never mentioned).
Comforters help Souls psychologically adapt and cope with new hosts and worlds (should this be needed). It is not clear whether they are truly the equivalent of psycholgists, but if they are this would mean they too are part of the health system, but unlike, Healers, deal with mostly mental instead mostly physical injuries.
Soul Medicinal Substances for Human Bodies
The Souls developed the following substances to help treat injuries to or other problems with human hosts.
A gas that helps stop infections when inhaled.
Stops infection of the wound it is applied to.
Encourages tissues to heal.
A dissolvable square placed on the tongue to bring down fever.
A shimmering powder that lessens the appearance of scars after Healing.
Cryotanks are generally a means of preserving organics, dead or alive, for long periods of time. In The Host, they are used to preserve Souls traveling from one planet to another, as they cannot survive without a host for more than a few minutes time.
Cryotank or cryogenic tank is a tank that is used to store frozen biological material.
The Souls travel from one planet to another by space ships referred to as shuttles. They often use it instead of aeroplanes.