Like their name suggests, opportunistic infections 'take the opportunity' when they have the chance. In more medical terms, they are infections that attack a person at a time when their body's immunity is below normal.
"We encounter germs on a daily basis. In fact, we have billions of microorganisms that live in and on us.
"But because of our immunity they cannot affect our health; we go on with life not knowing that our immunity is at work.
"The moment our immunity goes down, some of the germs that are in and on our body, will multiply and cause infection," says Dr Samuel Kinyanjui.
According to the medic, immunity can be lowered through several avenues. Use of steroids - which body builders have been documented to abuse sometimes - can cause immunity to go down.
"Prolonged starving too is another cause of lowered immunity. I am almost certain, that most Shakahola victims, had very low immunity at point of death," Dr Kinyanjui says.
Anti-cancer drugs (chemotherapy) can also lead to lowered immunity because as they attack and decimate cancer cells, normal healthy cells (immune cells) are not spared either.
Be that as it may, the most common (perhaps in Kenya) cause of lowered immunity is infection with human immunodeficiency virus (HIV).
Health & Science met Dr Kinyanjui on the sidelines of the Maisha Conference last month in Mombasa. He is the Country Director AIDS Healthcare Foundation (AHF Kenya) and is a specialist on HIV.
He says: "HIV lowers immunity by attacking white blood cells that contain a marker we call CD4. When a person's CD4 levels go down below 500 they start getting opportunistic infections.
"The commonest opportunistic infections are: Candidiasis (fungus that spread and cause oral thrush or vaginal thrush), toxoplasmosis, and tuberculosis (TB) - caused by bacteria that is usually already in our system."
When CD4 count goes below 350, advanced HIV infection sets in: at which point the doctor says one will definitely have developed TB. Advanced HIV infection leads to AIDS (Acquired immunodeficiency syndrome).
A patient of AIDS is prone to literally all possible infections that can afflict man. Luckily, most of the opportunistic infections, says Dr Kinyanjui, have specific treatment regimen.
Candidiasis can be treated using antifungals and there's a six-month treatment protocol for TB.
Permanent solutions to immunodeficiency lies with addressing the root cause of the problem.
In the case of HIV infection, one has to be put on antiretroviral drugs (ARVs) - whose dosage they must adhere to on a daily basis.
Anyone on steroids - that are not linked to treatment of a disease - would also need to stop using them.
As for cancer patients, they would need the advice of their doctor on how to go about the infections.