I don’t know why the word “infective” can’t be found in some dictionaries, can be found in some dictionaries, though.
Do you use this word “infective” often?
I know the words, infectious, infected and infection, but don’t know how to use infective, and wonder if this word does exist.
Thank you!!!
I think it is a word. Based on my knowledge and a little bit of research “Infective” refers to a property of an agent “like a disease” to be capable of being infectious. Generally speaking you don’t need to worry about it infectious is pretty much always a better word to use.
Never in my life have I ever heard (or read) the word “infective.”
Octo might have the origin of the word correct. As a non-native speaker, even as a native one unless you really know what you are doing, I would always use “infectious.”
Can confirm: as a native English speaker, I have never heard of the word ‘infective’ but would not be surprised if it has some technical meaning in the medical profession.
Just don’t get it mixed up with ‘ineffective’ 
I would strongly suggest always using “infectious.” The words are synonyms. The reason some dictionaries do not have “infective” is because it is vary rarely used word. The only time I would imagine someone using the word “infective” would be to avoid using the same word twice in the same sentence, or to try to sound sophisticated. (But would instead likely end up sounding pompous.)
The New Oxford American Dictionary of American English lists infective as a “dated” term, synonymous with infectious. However, the term infective is very much in use, in both its literal and figurative meaning.
Here are some common examples taken from Twitter and elsewhere:
(compound n.): infective endocarditis (The adjective infective is specifically used in the name of the disease, as part of the compound noun.):
If you get a staph or strep infection, this can lead to infective endocarditis, an infection of the inner linings of the heart. Bad oral hygiene can also cause this infectious disease, so brush your teeth, kids! (Staphylococcus and streptococcus are commonly shortened to staph and strep. Infectious disease is a very common term).
(n.) anti-infective (short for anti-infective drug):
Anti-infectives are a class of drugs that can suppress infection: antibiotics, antibacterials, antivirals, antifungals, antiparasitics, etc. Artemisinin is an anti-infective used to treat malaria. Discovered in 1972 by Chinese scientist Tu Youyou, who received the Nobel Prize for her discovery.
(adj.):
The common issue with bovine tuberculosis, which can jump the species barrier and cause tuberculosis-like infection in humans and other animals, is the contamination of the environment with the infective organism, Mycobacterium bovis, which poses a biosecurity risk to both cattle and wildlife. It’s time to stop seeing badgers as the “cause.” It is clear they are not.
(adj.) figurative:
Rebecca’s smile is infective.
Rubin’s laughter is very contagious and infective.
Two separate people told me today that my happiness was contagious and infective. That in itself makes me happy.
Thank you for inspiring others with your infective passion and enthusiasm.
Their defensive line came alive in the 2nd half and the energy was infective.
You’re probably more likely to hear:
Rebecca has an infectious laugh / an infectious smile.