In the movie It Comes At Night, who infected whom?

Unfortunately there is no true or real answer to this question. The director purposely left details vague and questions unanswered in an effort to keep "the fear of the unknown" theme going. So really, it is open to the viewers interpretation as to if Travis was the one who got Andrew sick or vice-versa.

"It was always important to me to never be a step ahead of the characters. We’re never going to find something out unless they find it out. I know what happened before the first frame, and how they got to that moment, all the way to what happens later, and certain things that aren’t explained. But the exciting thing to me is treating storytelling through the characters’ point of view, and really living it. And that’s going to frustrate a lot of people."

Here's another excerpt from an interview with the director Trey Edward Shults:

"The storytelling in the film is very deliberate, and what’s in there is intentional, and what’s not in there is intentional. I didn’t make a movie to frustrate you! I hope you can just be in the movie and let loose, and instead of it being a frustrating thing, it’s an interesting thing. And if you dig the movie, it’s something that you can return to and see new things, or analyze, or whatever you want to do."

[EDIT] And in that same interview, the director/writer also explains the originally intended (and longer) ending:

So there’s a whole other version of the ending that I have. It’s a final nightmare, basically, and these nightmares throughout are supposed to put you closer to what is going on inside Travis’ head and what he’s battling in his subconscious and get you closer to him, beyond just being scary or whatever. And there was a final one that was basically Travis’ reality has become a nightmare, so it’s all just a nightmare, and it was like a sort of dying fever dream where he confronts everything and it’s like his personal hell. And his hell is his house and his parents and facing what they’ve done.

Intellectually, it made a lot of sense, but in practice it was just pummeling and pummeling, and people weren’t with it emotionally. I found less is more, and I found a way to get across what I wanted to get across without beating people into submission.

This seems to suggest -- and the interview shows it a lot -- that the sickness is real for Travis at the end, wherever it came from. And it suggests that there's this kind of blur between nightmare and reality that comes from distrust and fear and maliciousness.

After repeating the last segment of the movie couple of times i have concluded Travis is the one to bring the infection inside the house.

Explanation:

Right after the table segment, where the families decides to not communicate with each other, Travis had a dream. In his dream he finds himself heading through the red door. Travis picks up a gun from the woods and heads deep eventually. Then he notices the dog bark in fear and also the presence of some other creature which might have infected the dog. This scene was again repeated right after he gets infected, while Travis lays in bed and he closes his eyes for the last time. These two scenes indicates that Travis was the one who Opened the door in his sleep to go to the woods to find the dog. Once found, He took it back home, where everyone finds it later. By this time Travis is awake and conscious and upon discovery he finds himself having to deal with the death of his dog.

Now for Andrew, kids always love to run around the house and sleep when they want where they want. The same thing happened to Andrew, and he ended up in grandpa's room. When Travis was conscious and curious, before the door was found open he touched Andrew and took him to his parent's room. This part definitely took place right after Travis rescued the dog with his subconscious mind as a result infecting himself. Andrew did not venture through the red door or touched the dog himself, as a kid he wont have the knowledge to wash his hand or wear gloves after a high possibility of blood stains from the dog.

PS. At the beginning of the movie Travis's mom was concerned if Travis after his grandpa's death, His parents also had a brief conversation about his physiological state that gave a vague idea about something being wrong about Travis. But you also did not realize the pictures that travis drew, thew where pictures of what he saw in the wood such as slenderman like drawings makes me believe that there's more than just human life in the area.

I believe that these so called (monsters) are the host of the virus but they come out at night to spread the disease or infection to kill of humans like the early english explorers but that's just a thought.

Once the film had finished, I had to wonder if Travis was infected all along. At the beginning of the film, the parents say something about Bud showing signs of infection within 24 hours, but what do we know about this infection? Almost nothing. For all we know, Bud had been hallucinating and sleepwalking for a while before he started to get boils and lesions.

Travis certainly isn't 'normal' - his nightmares (which made for truly horrifying cinema) and the drawings he drew in his spare time (which his father seemed unusually relaxed about) suggest some sort of psychological trauma, which probably goes back even further than witnessing his Grandfather's execution and funeral pyre.

We get a visual cue when we see one of his nightmares - the aspect ratio shifts and goes wider (or, if you're watching on a TV at home, narrower). If I recall correctly, the dreams are as follows

1. Travis walks through the red door to see his Grandfather seated and facing away from him. Bud roars and projectile vomits. His eyes are glassy and black (an exaggeration - Bud's eyes weren't like this at the beginning). He is awoken by his father who tells him someone is in the house.
2. Travis goes outside to check on the prisoner his father has tied to a tree. While looking for him by lamplight, Will suddenly appears in front of him with the same glassy black eyes as Bud's.
3. Travis dreams that Kim is sitting on the bed next to his (where his dog usually sleeps). She straddles him and vomits the black bile onto his face.
4. Travis goes looking for the dog in the woods. He takes a gun. He hears the dog bark, raises the gun but seems terrified by something else. There is a cut to (I think) him waking up in bed, and he begins vomiting the black bile. Then there's another cut and he wakes up again - for 'real' this time. He looks at his arms and they seem to be covered in boils. He rubs at them and eventually they disappear.
5. After Paul has killed Will's family, Travis is seen looking distressed. My memory is a little unclear here but I think then it goes into another dream sequence with his Grandfather with the black eyes. This time we do not jump awake but see from Travis' perspective his mother sat by his bed reassuring him.

As another user above suggested, I think events might be told slightly out of sequence with the real world. It seems most likely that in a fugue state, Travis snuck out of his parents room to look for the dog. He saw 'something' attacking the dog and managed to pull him away. He left the dog dying in the porch, then went back to bed. Then, he woke up, found Andrew in Bud's room, and returned him to his parents' room. He goes to check on the red door, and it is open. The outer door must also be open. Then he tells his father, and the rest of the film follows.

I'm pretty sure that's how events actually happened - his father has the only key and he was sleeping in the same room as him so would have been much more able to get it than Andrew would. This also seems to indicate that it's how he got infected. But that does throw into question all of the previous cues we have been given. The drawings, the nightmares, the placement of Kim in his dream, etc.

I think it's possible that the infection's early signs are hallucinations and sleepwalking, and the lesions come much later, right before the end. Therefore, Andrew may also be infected.

I totally thought that is what Travis's last vision meant. He opened the door and lied about it, or blocked it out. Will and his family wanted to leave because they new Travis lied and was now sick.

If the grandfather died because he had virus, there's a good chance all of the remaining family members were infected because they lived in the same house. The premise that you can only be infected by touch is ridiculous...if that was the case then everyone is infected because everyone touched one or more times during movie. Remember when families met? Everyone shook hands.

The writer's were purposely depicting Travis as a disturbed young man throughout the movie. I think this was intentionally done so all unanswered questions the audience had would lead to Travis. Door being unlocked could not have been done by child. He's too short. Movie does make alot of sense.

We are not given details of how Bud contracted the infection, but it's significant that a member of the family home had already contracted the disease: it means the pathogen was already in the home.

While it's significant that Andrew was found sleeping on the floor of Bud's room, the movie never gives real evidence that Andrew was infected. Instead, its Travis who show clear signs of being infected first. So who infected him?

My theory is that the dog is the source of the infection. Throughout the movie we see the dog being allowed to roam free inside or outside. This was the crucial mistake that doomed everybody within home, despite the extreme precautions taken.

In fact, Bud's original infection may have been brought in by the dog originally, but we aren't given enough information to know.

Either way: Travis got sick before everyone else, and the only behaviour he had over the rest of his family was sleeping closely with the dog every night. If Bud's leftover pathogen had spread to the dog or its fur, then Travis was clearly likely to be the next victim.

When the dog began barking erratically and ran away from the home (likely just chasing an animal, but being disobedient in manner), it was merely the dog showing symptoms of being infected similar to the strange dreams and thoughts shown by Travis. As the infection flourished within both, we can explain the unlocked door as Travis acting erratically. His "dream" of going outside at night was real, and when he returned he did not leave the door locked as he should have, leading to the sick dying dog returning to the red door infected with wounds (possibly) made worse by an animal attack.

I believe like a few others have already said. The dog and the boy was the cases that followed the grandpa. Dog tripping out in the woods and hallucinations of Travis was side effects of the virus. The pictures the boys was drawing are kind of creepy, but I think its of them being infected and seeing themselves as monsters. Also I feel like the picture Travis drew and his dad actually looked at, to me it looked like the men he killed that had attacked him and other guy going to rescue his family. But was such a fast screen .