[ConfusedHermit (OP)]

When our body overcomes an illness, is that illness gone forever?

If we were to become less healthy and our immune system weakened, would that illness still be there-- and because we'd be weaker then, could it affect us?


The example in my head would be a person with a terrible disease, who miraculously survives or 'beats' it by being so healthy, but later becomes unhealthy and the disease (having never left their body or truly 'beaten,' just delayed) is now stronger than they can survive from.

[wolfekeeper]

Many illnesses your body can overcome completely, and once you recover, it's gone.

But some illnesses, your body can't completely eradicate; but your body will have immunity to it, so even though it's still lurking, it won't do anything.

For example chicken pox, when you recover, the disease continues to hide in your nerves and can later come back in the form of shingles.

Another illness is toxoplasmosis, this is a parasitic disease caught from badly cooked food. Once you get it, you get a flu-like illness, from which you usually recover quite well. However, it's still there, hiding, and whenever it pops up the immune system takes it out. If you get an immune system problem (AIDS being the classic example) the effects of the latent toxoplasmosis can then be fatal.

[ConfusedHermit (OP)]

Thank you very much :]

[CliffordK]

Some bacteria such as Staph Aureus and Streptococcus will colonize a portion of the population, and is eradicated from another portion of the population.  And, thus, it is continues to survive in the population.

Smallpox was one of the early eradication successes because it does not colonize the infection survivors.  So, once all pockets of wild disease were wiped out, the disease also was wiped out.

AIDS is extremely hard to combat because it is difficult to eradicate the disease from the body, and follows a slow progressive path.

A country is apparently considered Polio-Free if it has no cases for two years, indicating no long-term colonization.  However, there are two polio vaccines.  OPV is the most effective single dose vaccine which uses a live attenuated virus, but there is a low rate of vaccine associated illness.  IPV is a "dead virus", and thus can not cause infection.  However, it requires multiple injections which are only practical if there is a good immunization system in the country.  It is likely that Polio will be eradicated within our lifetimes.

As Wolfekeeper mentioned, Chickenpox (varicella) will endure in the nervous system of essentially all individuals who have been infected for the life of the individual.  It is not generally infective during the dormant stage, but becomes infective again if the individual develops shingles (not all individuals).  Thus, it will take at least a century to eradicate the disease using vaccines.  One of the concerns is that periodic exposure to wild chickenpox may boost individual's immunity, and prevent the later occurrence of shingles.  By vaccinating all the children, the elderly may get less exposure to chickenpox (and periodic immunity boosts), and it may lead to more cases of shingles (thus, there are recommendations to give chickenpox boosters to individuals who had previously had chickenpox and are at risk for shingles).

Herpes generally will colonize individuals for life.

Hepatitis is a term for several different viral infections that affect the liver. 
Hepatitis A is generally an acute infection, and is readily transmitted via fecal-oral.
Hepatitis B&C can have both an acute and a chronic phase (but not always a chronic infection), but essentially require closer contact with transmission via bodily fluids.

Another important distinction is whether there are animal hosts for the disease, or how long it can survive in nature.
Rabies is difficult to eradicate because of important wild animal hosts.

Malaria is transmitted by mosquitoes, but generally requires a human host.  Without active human infections, it would die out.  It can be carried by northern mosquitoes, but it is much less infectious in northern climates because during the winter, mosquitoes are unable to pass the disease from individual to individual, and the disease dies out.  In tropical regions, the mosquitoes can continue to propagate the disease year around.

Lice and Fleas are generally species specific, but can readily re-infect individuals that have eliminated the infection.

Trichinosis can infect humans through consumption of infected meat (pork).  It can then stay with the human for the rest of their lives, but is generally not infectious (assuming the host flesh isn't eaten).

Intestinal worms can also cause long-term colonization of human hosts, but can be eradicated with various de-worming agents.

[cheryl j]

Type II diabetes is kind of interesting. If a person changes his diet, loses weight, exercises, and gets his blood sugar consistently in the normal range before any permanent damage is done to blood vessels or organs is he still "diabetic"? As far as I know, there is no way a doctor could tell without knowing his medical history, although the patient still may have a genetic predisposition.

There are other diseases that seem to surface when the person is under stress and subside when conditions change.

[evan_au]

There are genetic reasons why some disease immunity becomes obsolete - the genetics of the disease can change.

Once a significant fraction of the population is immune to one strain of the disease, mutations which affect the outer coat of the virus or bacteria (without reducing the infectivity) will then rapidly spread through the population, because people's immune systems don't recognise them as "dangerous".

HIV is an "RNA virus", with a single strand of genetic material. Because it only has a single copy of the genetic material, and doesn't have the error-checking mechanisms of human DNA, mutation rates are high, and HIV quickly overcomes most single medications.

Influenza is also an RNA virus, but it also has 8 strands of RNA. So it can easily "cross-breed" with other strains of influenza, producing regular new strains which sweep around the world every year.

[schneebfloob]

Mycobacterium tuberculosis is another one that has the potential to linger for many years (potentially your lifetime). The majority of people who have been infected with Mycobacterium tuberculosis will be latently infected, following the primary active infection.  In most people this won't come to anything, but in about 10% of cases it will re-emerge (becoming more likely in patients that have AIDS).

I have read estimates from the WHO that approximately a third of the population of the planet are infected with latent TB, the majority of whom will be in the developing world.