Derek Broughton wrote:

Giardia isn't a deadly
disease, afaik, but you never get rid of it.

Giardia can be deadly and you can get rid of it.

Giardia causes severe dehydration which can be deadly if not treated. A
friend of mine got giardia while hiking the Appalachian Trail and almost
died before he could get to a hospital. He was hospitalized for a while.

Giardia can be cured by treatment with drugs such as metrodiazole
(Flagyl) or nitazoxanide. If a single cyst remains in the body,
reinfection can occur. In this case further treatment is necessary.
Some doctors have other courses of treatment for cases that are very
difficult to cure.

The disease is caused by cysts and trophozoites that are a type of
giardia, a protozoan flagellate. A single cyst can cause an infection.

The infection is called giardiasis. Giardiasis (GEE-are-DYE-uh-sis) is
a diarrheal illness caused by a one-celled, microscopic parasite,
Giardia intestinalis (also known as Giardia lamblia ). Once an animal or
person has been infected with Giardia intestinalis , the parasite lives
in the intestine and is passed in the stool. Because the parasite is
protected by an outer shell, it can survive outside the body and in the
environment for long periods of time. During the past 2 decades,
Giardia infection has become recognized as one of the most common causes
of waterborne disease (found in both drinking and recreational water) in
humans in the United States.

If one must drink water of uncertain sources, boiling kills the parasite
instantly. You do not have to boil the water for 5-10 minutes. As soon
as the water comes to boil, the parasite is killed.