We can be sure of this because humans, like many other species, are hosts to a number of obligate, species-specific, parasitic pathogens, i.e. parasites which are obliged to live in or on their host in order to survive.
Chlamydia trachomatisChlamydia trachomatis is an obligate intracellular pathogen (i.e. the bacterium lives within human cells) and can cause numerous disease states in both men and women. Both sexes can display urethritis, proctitis (rectal disease and bleeding), trachoma, and infertility. The bacterium can cause prostatitis and epididymitis in men. In women, cervicitis, pelvic inflammatory disease (PID), ectopic pregnancy, and acute or chronic pelvic pain are frequent complications. C. trachomatis is also an important neonatal pathogen, where it can lead to infections of the eye (trachoma) and pulmonary complications. C. trachomatis is the single most important infectious agent associated with blindness; approximately 600 million worldwide suffer C. trachomatis eye infections and 20 million are blinded as a result of the infection.
SyphilisSyphilis is a sexually transmitted infection caused by the spirochete bacterium Treponema pallidum pallidum.T. pallidum pallidum is a spiral-shaped, Gram-negative, highly mobile bacterium (see electron micrograph - left). Three other human diseases are caused by related T. pallidum subspecies, including yaws (T. p. pertenue), pinta (T. p. carateum) and bejel (T p. endemicum). Unlike T. p. pallidum, they do not cause neurological disease. Humans are the only known natural reservoir for T. p. pallidum. It is unable to survive without a host for more than a few days. This is due to its small genome (1.14 MDa) and thus its inability to make most of its macronutrients. It has a slow doubling time of greater than 30 hours.
Source: Wikipedia - Syphilis
GonorrhoeaGonorrhoea is a common human sexually transmitted infection caused by the bacterium Neisseria gonorrhoeae. It is unique to humans.
The infection is transmitted from one person to another through vaginal, oral, or anal sex. Men have a 20% risk of getting the infection from a single act of vaginal intercourse with an infected woman. The risk for men who have sex with men is higher. Women have a 60–80% risk of getting the infection from a single act of vaginal intercourse with an infected man. A mother may transmit gonorrhea to her newborn during childbirth; when affecting the infant's eyes, it is referred to as ophthalmia neonatorum. It cannot be spread by toilets or bathrooms.
Source: Wikipedia - Gonorrea
And every living substance was destroyed which was upon the face of the ground, both man, and cattle, and the creeping things, and the fowl of the heaven; and they were destroyed from the earth: and Noah only remained alive, and they that were with him in the ark.
If this were true, if would mean that members of Noah's family were carries of these venereal and sexually transmitted diseases.
Would any creationist like to speculate on who they might have been?
And the Lord said, I will destroy man whom I have created from the face of the earth; both man, and beast, and the creeping thing, and the fowls of the air; for it repenteth me that I have made them. But Noah found grace in the eyes of the Lord.
These are the generations of Noah: Noah was a just man and perfect in his generations, and Noah walked with God. And Noah begat three sons, Shem, Ham, and Japheth.
So that's four men and four women. Did just one of them carry all three sexually transmitted diseases or were they spread around? We can be fairly sure that their partners would have been infected too so at least two and maybe as many as six of the humans on the Ark had sexually transmitted diseases normally, though not always, the result of having several sexual partners. One thing we know is that you can't catch them from virgins unless they had a congenitally acquired form acquired from their mother, so we can be fairly certain that one or more of the people on the Ark had had extramarital sexual relations of some sort.
So, is this yet another example of an unintelligent god who hasn't thought things through? In a fit of pique it decides to destroy everyone and everything because they are sinners, then realises it needs to save sinners too in order to save the diseases it's also created, so negating the entire purpose of the whole multi-ethnic, multi-species genocide, but it does it anyway.
Or is it just a nonsense tale made up by people who were in complete ignorance of bacteriology and microbial causation of disease?